I've been trying to learn the science of the sun and the aurora as a hobby for a couple of months now and I've recently come across the term "Russell-McPherron effect" as for the reason why people say the aurora is better around the equinoxes. I can't seem the wrap my head around it too well and I've tried to research it but I am either not as good at researching as I think or there isn't much information on it. Is there anyone here that can understand it or have researched it before?

Welcome Kianna,

The term is new to me but as far as I know aurora is statistically slightly more common during the equinoxes. I did hear and read about this yes. Not sure if there is any solid research done on this as to why this is but I assume Earth's tilt (and thus the orientation of magnetic field in relation to the incoming solar wind) plays a role in this.

I don’t fully understand it either. Here is what spaceweather.com says:

Disclamimer: this may not be fully accurate. Do your own research. 
It's true. Auroras really do love spring. Researchers call it the "Russell-McPherron effect." During the weeks around equinoxes, cracks form in Earth's magnetic field, allowing solar wind to enter. Even a weak stream of solar wind or an off-target CME can spark a good display at high latitudes.

Edited March 23, 20223 yr by Orneno

I get a rash every time they talk about ''cracks'' in earth's magnetic field... 🤣

Still, the science behind why activity is more common during equinoxes is not understood as far as I know.

1 minute ago, Marcel de Bont said:

I get a rash every time they talk about ''cracks'' in earth's magnetic field... 🤣

Oh phew, I’m glad I’m not the only one who raises their eyebrow when they talk about some southward Bz in a CME being a crack in Earth’s magnetic field 

We bandied about this topic last year for a bit - not the scientists' names attributed to the pheonomenon but the phenomenon itself:  why more geomagnetic storms (and aurora) occur around the equinoxes.

2057-are-impacts-stronger-in-summer

Newbie: "The affect of the sun on the earth has more to do with the orientation of the Earth's magnetic field at equinox. During autumn and spring equinox, this orientation enables a 'connection' to the sun thereby allowing solar wind to stream in, it is not the case at other times of the year."

Edited March 23, 20223 yr by Drax Spacex
URL

Thank you everyone for replying! While I was trying to search around for it, I did get the idea that it wasn't studied very well yet, so I'm happy someone at least replied to this.

Edit: I just read the post that you linked, Drax, and that is exactly what I was looking for! It helped me understand the phenomenon a little easier.

Edited March 24, 20223 yr by Kianna

I wonder if the recent (quite nice) displays on Feb 18 and 19 (US Eastern time) had something to do with the RMP effect? On Feb 19 ("close" to equinox), a nice aurora show was seen on Sebec Lake aurora web cam (latitude 45). I am pretty sure they were visible from the entire US northeast on that night. Yet, the spaceweather indices were not all that great: Kp 2.67, the magnetometers in Canada barely budged (Ottawa was flatlined, for example), Bz fluctuated between -2 and -5nT, wind speed between 450 and 480 km/s, and density between 5 and 15, roughly, HPI was almost steady at 50GW for 5 straight hours and the display occurred at the end of this period and lasted for about 20 minutes.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012JA017845

Edited February 23Feb 23 by JessicaF
Added a link to a relevant study

On 2/23/2025 at 7:06 AM, JessicaF said:

I wonder if the recent (quite nice) displays on Feb 18 and 19 (US Eastern time) had something to do with the RMP effect? On Feb 19 ("close" to equinox), a nice aurora show was seen on Sebec Lake aurora web cam (latitude 45). I am pretty sure they were visible from the entire US northeast on that night. Yet, the spaceweather indices were not all that great: Kp 2.67, the magnetometers in Canada barely budged (Ottawa was flatlined, for example), Bz fluctuated between -2 and -5nT, wind speed between 450 and 480 km/s, and density between 5 and 15, roughly, HPI was almost steady at 50GW for 5 straight hours and the display occurred at the end of this period and lasted for about 20 minutes.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012JA017845

Hard to say for sure, but could be. Despite the lack of Kp (which isn't always that great for determining more localized aurora visibility) there was what looks like a clear substorm towards the end of Feb 18:

The day after, Feb 19, also shows some disturbances:

Looking at the solar wind parameters the field was oriented "towards", which is indeed what you expect to be associated with increased activity near the March equinox as per the R-M effect, but I doubt that alone was the cause, as the field was oriented in the same way for quite some time both before and after it without notable activity. Rather there seems to have been some disturbance in the solar wind, or maybe just a streak of favorable conditions by chance, leaving the Bz moderately negative for quite a few hours in a row:

The density had also been a bit higher than average for a while by the looks of it, and the speed towards the higher end of what's normal for the solar wind too. Looking at SDO imagery I don't think there were any CHs involved, and I don't think it was any CME action either. As per the synoptic map for Feb 17 we were passing over some ARs and some spotty fields, which could perhaps have something to do with the disturbance; there were some small fluctuations in the phi angle at that time, as you can see.

Overall I would say it probably played a minor role given the "towards" field at the time, but was likely not the main culprit. It bears remembering that the R-M effect itself has peaks a couple of weeks after the equinoctes, so Feb 18-19 would still not see a major contribution from it, but still some. As shown in the paper you linked to it also has a diurnal component, and that one actually does match up fairly well with the time of the disturbance and the substorm, so that points a bit more in favor of a contribution too.

The separate effect also mentioned in the paper, the equinoctial effect, is generally assumed to have a stronger effect, and peaks right at the equinoctes, so it would probably contribute a bit more. It also has to do with the component of the speed perpendicular to the field, so the higher the speed, the more it matters; in this case with the slightly elevated speed that would have led to a somewhat higher contribution from it.

As always it's often an interplay of many different factors with varying contributions that come together to make for activity, and this disturbance, while fairly minor, is no exception!

Edited February 27Feb 27 by Philalethes
typo

1 hour ago, JessicaF said:

My final question is, given the positive By for this event and given the season, did the seasonal variation and perhaps the R-M effect play some role here? I need to make an effort to remember which way the Earth rotational axis tilts during the spring equinox (when viewed from x, the direction from the sun). Is it left or right?

It's doubtful that the R-M effect played any role in that particular scenario, as the field was mostly "away" due to the positive polarity of the CH(s) in question, hence also the generally positive By. Around the March equinox (peaking a couple of weeks after, beginning of April), the R-M effect essentially states: when the magnetic field of the sun is "away", which means we see a positive By and a negative Bx, then the Bz is more likely to be positive, and vice versa, when the magnetic field of the sun is "towards", showing a negative By and a positive Bx, then the Bz is more likely to be negative (which is what we want, eh?); for the September equinox it's the opposite, with "away" yielding a higher chance of a negative Bz, and "towards" a higher chance of a positive Bz.

Note that the Bx and By themselves don't really matter at all, and that the By's role in the Newell coupling function is something completely different; they're just indicators of which way the magnetic field of the sun is oriented, i.e. either away from or towards the surface. It's the increased or decreased likelihood of negative or positive Bz which really matters.

And yeah, remembering which way Earth is tilted is a bit like relearning left and right from scratch, heh. My suggestion for a heuristic would be to remember the much simpler fact that Earth orbits the sun in a counterclockwise direction. That way you can more easily envision Earth's tilt at the solstices, and just move it a quarter of the way along its orbit to the appropriate equinox.

In your case, considering the March equinox (at least I assume you mean the spring equinox in the NH, as that's what's usually meant), simply consider which way it's tilted during the preceding solstice, i.e. the December solstice; the South Pole would be tilted towards the sun and the North Pole away from it, so after orbiting a quarter of the way to the March equinox the North Pole would be tilted in the clockwise direction of the orbit, and the South Pole in the counterclockwise direction that Earth is moving in. So if you were standing on the solar surface and looking at Earth, the North Pole would be tilted to the right, and the South Pole tilted to the left.

Alternatively just keep an image like this around that you can reference until it sticks, heh:

But of course, while all of this is very interesting for those who like to read about it, it can be reduced to fairly simple heuristics when it comes to aurora:

• Near April 6, you want the field to be "towards", so negative coronal holes are great.

• Near October 10, you want the field to be "away", so positive coronal holes are great.

That's pretty much the gist of it. It also applies to the regular solar wind, so you can look at the daily synoptic maps and see which direction the field will be in around those times and see if you can make out the association; typically the regular solar wind will of course be weaker and not lead to too much activity, but if you e.g. pass across a wide area of field of correct polarity it can definitely help to prime the magnetosphere by bringing about e.g. Kp3 and Kp4 conditions, even minor geomagnetic storming in some cases.

Edited March 1Mar 1 by Philalethes
quote formatting

1 hour ago, Philalethes said:

It's doubtful that the R-M effect played any role in that particular scenario, as the field was mostly "away" due to the positive polarity of the CH(s) in question, hence also the generally positive By. Around the March equinox (peaking a couple of weeks after, beginning of April), the R-M effect essentially states: when the magnetic field of the sun is "away", which means we see a positive By and a negative Bx, then the Bz is more likely to be positive, and vice versa, when the magnetic field of the sun is "towards", showing a negative By and a positive Bx, then the Bz is more likely to be negative (which is what we want, eh?); for the September equinox it's the opposite, with "away" yielding a higher chance of a negative Bz, and "towards" a higher chance of a positive Bz.

Note that the Bx and By themselves don't really matter at all, and that the By's role in the Newell coupling function is something completely different; they're just indicators of which way the magnetic field of the sun is oriented, i.e. either away from or towards the surface. It's the increased or decreased likelihood of negative or positive Bz which really matters.

I am not sure I am completely clear on the R-M effect and the seasonal effect. Is the seasonal effect something else or is it just a different name for R-M?

The intuition I started with seems natural but is probably wrong - one would think that the "best" orientation of the IMF is exactly the opposite of the Earth's dipole. However, from what you write the only thing that really matters is the negative component perpendicular to the ecliptic. I always thought this was just an approximation as setting up a coordinate system with the Earth's axis as z would be too cumbersome. Hence, I believed that in the spring we want By < 0 as the IMF projects onto Earth's axis south and vice versa in the fall. So, this is not correct?

Also, when you say negative polarity CH, do you mean negative By? Just making sure I understand.

BTW, this was just posted on spaceweather.com: (are you familiar with Hathaway's study?)

MARCH IS THE BEST MONTH OF THE YEAR FOR AURORAS: Did you know that March is the best month of the year for auroras? It's true. A 75-year study by retired NASA solar physicist David Hathaway shows that March has more geomagnetically active days than any other month of the year. (October is a close second.)

According to historical records, geomagnetic disturbances are almost twice as likely in spring and fall vs. winter and summer. This is due to the Russell-McPherron effect. In short, cracks tend to form in Earth's magnetosphere during weeks around equinoxes, allowing solar wind to spark Northern Lights. Happy March!

Hi, I was only yesterday looking into the Russell McPherron effect. During the equinoxes, the Sun is directly in line with the equator on Earth giving the entire planet the same amount of daylight hours. Usually, Earth's magnetic field lines and those of the solar winds, are misaligned. The Russell McPherron effect is when the magnetic field lines from Earth align with those of the solar wind. So if the IMF hits the magnetosphere at exactly the centre of Earth's magnetic lines then more of the Sun's magnetic field is going to flow through Earth's field lines to the poles. so it is not essential to have such a strong IMF to trigger substorms and auroral activity is more intense. Hope this helps x

20 minutes ago, JessicaF said:

I am not sure I am completely clear on the R-M effect and the seasonal effect. Is the seasonal effect something else or is it just a different name for R-M?

Don't fret, it can be quite tricky, took me quite some time to piece together how it works. There are essentially two separate effects which come together to cause the increase in geomagnetic activity around the equinoctes each year. One of them is the R-M effect, which operates in the manner described thus far; the other is the so-called equinoctial effect, which has to do with the component of the solar wind speed that's perpendicular to the geomagnetic dipole. The latter of these is generally considered the stronger effect, and the primary reason for the higher geomagnetic activity, whereas the R-M effect is generally considered a smaller part of it, but under the right conditions it can really shine. Of note is that the equinoctial effect is exclusively based on Earth's tilt, so it has its peaks directly on the equinoctes, whereas the R-M effect also depends on the solar axial tilt, peaking a couple of weeks after each equinox.

The distinction can be quite confusing, because it seems like many people have ended up simply using the R-M effect as a catch-all term for the increased activity around the equinoctes, despite how it's likely the smaller of the two effects playing a role.

26 minutes ago, JessicaF said:

The intuition I started with seems natural but is probably wrong - one would think that the "best" orientation of the IMF is exactly the opposite of the Earth's dipole. However, from what you write the only thing that really matters is the negative component perpendicular to the ecliptic. I always thought this was just an approximation as setting up a coordinate system with the Earth's axis as z would be too cumbersome. Hence, I believed that in the spring we want By < 0 as the IMF projects onto Earth's axis south and vice versa in the fall. So, this is not correct?

That intuition is indeed slightly wrong, but very close to correct. Due to the direction of the solar wind itself being radial, what ultimately matters is the component of Earth's dipole in the yz-plane (which is how the GSM coordinate system is defined); but for that component the best orientation is indeed exactly opposite it, i.e. negative Bz and zero By (yielding 1 for the sine of the clock angle, the maximum value). It's not relative to the ecliptic itself, but the dipole's component in that yz-plane (that plane itself is the same for all the geocentric coordinate systems where the x-axis is radially towards the sun, i.e. GSM, GSE, and GSEQ). The GSM coordinate system used for the solar wind, i.e. the Bx, By, and Bz, is specifically relative to the dipole's projection into that yz-plane, and continuously changes both annually and diurnally (the diurnal change due to how the geomagnetic poles are offset ~10° from the geographic poles and thus rotate around them each day).

It's thus partially correct that we want a negative By (and a positive Bx) in the spring; not because the By itself matters to the projection, but simply because it's associated with a higher likelihood of a negative Bz as per the R-M effect, since the field is "towards". For e.g. a CME this association goes out the window, and it doesn't matter at all what the direction of the By is even near the R-M peaks, because the By will no longer have that association that the regular solar wind has as it carries the surface field out.

43 minutes ago, JessicaF said:

Also, when you say negative polarity CH, do you mean negative By? Just making sure I understand.

In a way that's true, though what I really mean is a CH situated in a region of the sun that has a negative polarity, i.e. where the magnetic field is directed inwards into the solar surface; but when this surface field is carried out by the solar wind it will indeed be associated with a negative By at L1 (and a Bz-component depending on the time of the year, more likely positive around the beginning of April, and vice versa around the beginning of October, the crux of the R-M effect).

46 minutes ago, JessicaF said:

are you familiar with Hathaway's study?

No, I don't think I am; and I see they didn't post it there either. Tried searching for it, but can only find articles referring back to there, not the actual study. The findings are however pretty much exactly what you'd expect, and that geomagnetic activity is higher around the equinoctes is certainly not news, heh; but I guess quantifying exactly which months see the most activity is interesting, and might uncover some details about it.

I posted an updated version of this recently, maybe you saw it, but it's very obvious at a cursory glance that activity is elevated during those parts of the year:

Edited March 1Mar 1 by Philalethes
coordinate system clarification

1 hour ago, Philalethes said:

Don't fret, it can be quite tricky, took me quite some time to piece together how it works. There are essentially two separate effects which come together to cause the increase in geomagnetic activity around the equinoctes each year. One of them is the R-M effect, which operates in the manner described thus far; the other is the so-called equinoctial effect, which has to do with the component of the solar wind speed that's perpendicular to the geomagnetic dipole. The latter of these is generally considered the stronger effect, and the primary reason for the higher geomagnetic activity, whereas the R-M effect is generally considered a smaller part of it, but under the right conditions it can really shine. Of note is that the equinoctial effect is exclusively based on Earth's tilt, so it has its peaks directly on the equinoctes, whereas the R-M effect also depends on the solar axial tilt, peaking a couple of weeks after each equinox.

The distinction can be quite confusing, because it seems like many people have ended up simply using the R-M effect as a catch-all term for the increased activity around the equinoctes, despite how it's likely the smaller of the two effects playing a role.

That intuition is indeed slightly wrong, but very close to correct. Due to the direction of the solar wind itself being radial, what ultimately matters is the component of Earth's dipole in the yz-plane (which is how the GSM coordinate system is defined); but for that component the best orientation is indeed exactly opposite it, i.e. negative Bz and zero By (yielding 1 for the sine of the clock angle, the maximum value). It's not relative to the ecliptic itself, but the dipole's component in that yz-plane (that plane itself is the same for all the geocentric coordinate systems where the x-axis is radially towards the sun, i.e. GSM, GSE, and GSEQ). The GSM coordinate system used for the solar wind, i.e. the Bx, By, and Bz, is specifically relative to the dipole's projection into that yz-plane, and continuously changes both annually and diurnally (the diurnal change due to how the geomagnetic poles are offset ~10° from the geographic poles and thus rotate around them each day).

It's thus partially correct that we want a negative By (and a positive Bx) in the spring; not because the By itself matters to the projection, but simply because it's associated with a higher likelihood of a negative Bz as per the R-M effect, since the field is "towards". For e.g. a CME this association goes out the window, and it doesn't matter at all what the direction of the By is even near the R-M peaks, because the By will no longer have that association that the regular solar wind has as it carries the surface field out.

In a way that's true, though what I really mean is a CH situated in a region of the sun that has a negative polarity, i.e. where the magnetic field is directed inwards into the solar surface; but when this surface field is carried out by the solar wind it will indeed be associated with a negative By at L1 (and a Bz-component depending on the time of the year, more likely positive around the beginning of April, and vice versa around the beginning of October, the crux of the R-M effect).

No, I don't think I am; and I see they didn't post it there either. Tried searching for it, but can only find articles referring back to there, not the actual study. The findings are however pretty much exactly what you'd expect, and that geomagnetic activity is higher around the equinoctes is certainly not news, heh; but I guess quantifying exactly which months see the most activity is interesting, and might uncover some details about it.

I posted an updated version of this recently, maybe you saw it, but it's very obvious at a cursory glance that activity is elevated during those parts of the year:

I am embarrassed to say that until now, I thought the GSM coordinates had the z axis pointing perpendicularly to the ecliptic. It makes sense in celestial mechanics but not for solar/magnetosphere matters. I see now that z points in the direction of the dipole axis (https://www.ngdc.noaa.gov/geomag/gsm2geo.shtml#:~:text=GSM%20Coordinate%20System&text=The%20origin%20is%20defined%20at,x%2D%20and%20y%2Daxes.). Just this fact on its own explains a lot things! This means that the z axis wobbles diurnally by 10 degrees with respect to the rotational axis.

So, if I understand correctly, what needs to be considered are the prevailing polarities of sun's surface magnetic field - they are not random and have preferences - and it is these preferences that combine with the Earth's magnetic field better at equinoxes than solstices that is the seasonal (and R-M) effect? If I am wrong and the surface polarities can be equally likely in all directions, then I am LOST.

BTW, on spaceweather.com, Tony seems to claim in his latest article on the seasonal variation that it is due to the R-M effect. This is what you meant by people bundling both effects into one, calling it R-M? Your chart of seasonal variability indeed shows a very strong dependence.

6 minutes ago, JessicaF said:

I am embarrassed to say that until now, I thought the GSM coordinates had the z axis pointing perpendicularly to the ecliptic. It makes sense in celestial mechanics but not for solar/magnetosphere matters. I see now that z points in the direction of the dipole axis (https://www.ngdc.noaa.gov/geomag/gsm2geo.shtml#:~:text=GSM%20Coordinate%20System&text=The%20origin%20is%20defined%20at,x%2D%20and%20y%2Daxes.). Just this fact on its own explains a lot things! This means that the z axis wobbles diurnally by 10 degrees with respect to the rotational axis.

It's tricky stuff, definitely nothing to be embarrassed about. And yeah, the coordinate system with the z-axis perpendicularly to the ecliptic would be the GSE coordinate system (geocentric solar ecliptic); this is used more for the propagation of solar wind and CMEs and such (while the magnetic field lines are bent as per the Parker spiral, the solar wind itself actually flows radially outwards, unless deflected for some reason).

As for GSM, there's an important thing to note about it that you might still have missed; as the page reads:

The y-axis is defined as the cross product of the GSM x-axis and the magnetic dipole axis; directed positive towards dusk. The z-axis is defined as the cross product of the x- and y-axes. The magnetic dipole axis lies within the xz plane.

That's a mouthful, but what does that really mean? It means that that the z-axis in GSM does not point in the direction of the dipole axis after all, but it points in the direction of the dipole axis projected onto the yz-plane. Since the x-axis is radially towards the sun in all of these coordinate systems you can probably envision what is meant by the yz-plane, which is also the same for them; the only difference between the coordinate systems is the different directions that the y- and z-axes point in. So if you consider that plane, and then think about the dipole axis, it will readily become apparent that it most of the time will not lie in this plane, but rather have some component along the x-axis as well.

For GSM coordinates you essentially just project that dipole onto the yz-plane, as if casts a shadow onto it, so to speak. You ignore the x-component of the dipole, then take the y- and z-components of it and calculate the angle between their combination and some reference (GSE is useful for making that transformation, since you can easily define the axial tilt and the dipole tilt relative to it). This is all what taking the stated cross product simplifies, making it into a fairly simple calculation. In the end you get the angle between the y- and z-axes in the reference (like GSE) and the GSM system.

48 minutes ago, JessicaF said:

So, if I understand correctly, what needs to be considered are the prevailing polarities of sun's surface magnetic field - they are not random and have preferences - and it is these preferences that combine with the Earth's magnetic field better at equinoxes than solstices that is the seasonal (and R-M) effect? If I am wrong and the surface polarities can be equally likely in all directions, then I am LOST.

Well, there are ultimately only two polarities, which are quite prevalent indeed, heh; they cover the entire surface, in fact! The field is divided into the two polarities, positive and negative (outwards and inwards respectively). I mean, this of course isn't anything new, or that you don't really already know, obviously, but what it means isn't as obvious.

Take e.g. the most recent synoptic map as of writing this:

Here you can see some of the larger divisions into the two polarities, with coronal holes always (at least to my knowledge) falling inside field of a single polarity, as the field lines there are all in the same direction and extending out farther than others. So for the positive regions (+) the field is outwards, and for the negative ones (-) the field is inwards, and the field lines are also bent clockwise as per the Parker spiral; it's this bending which gives the field the components associated with the R-M effect. The solar wind is essentially carrying out this field, so when solar wind hits us the IMF of it will tend to be what it was when the wind left the surface.

Due to this, the magnetic field of solar wind will broadly fall into one of the two categories when it hits Earth. Either it will have a positive By and a negative Bx, resulting in a phi angle of ~135°, or it will have a negative By and a positive Bx, resulting in a phi angle of ~315°. These correspond to "away" and "towards" respectively, and is the reason why the RTSW data for the phi angle is marked the way it is; zooming out across a whole year you can see how we cross into and out of the two polarities, broadly speaking:

If you look at the image I drew on and posted here, then maybe it will become clearer. Since the field lines are bent, they will either be in the direction with or against the axial tilt of Earth at the equinoctes depending on their polarity. The By itself, like the Bx, isn't really relevant for the geomagnetic activity itself there, it's just to show that when the field is directed that way, you end up with a By in a certain direction, and for the same reason you tend to end up with a Bz in a certain direction. I will post the image here too for reference:

Hopefully you can see that at the September equinox, the red arrow is going outwards ("away") and against the tilt, which tends to give a negative Bz component; the blue arrow, on the other hand, goes with the tilt, and tends to give a positive Bz component. At the March equinox it's the other way around, where the blue arrow "towards" goes against the tilt, and thus is the one more likely to produce negative Bz.

That is all only the R-M effect. What you call "the seasonal effect", also called more generally "semiannual variation of geomagnetic activity", is a combination of that R-M effect and a separate effect that doesn't have anything to do with what we're talking about here at all, heh. That other effect has to do with the solar wind speed perpendicular to the dipole, and is called the equinoctial effect. It's also widely understood to be the most significant of the two, accounting for most of the increased activity near the equinoctes.

1 hour ago, JessicaF said:

BTW, on spaceweather.com, Tony seems to claim in his latest article on the seasonal variation that it is due to the R-M effect. This is what you meant by people bundling both effects into one, calling it R-M? Your chart of seasonal variability indeed shows a very strong dependence.

If he ascribes all the seasonal variation to that, then yes, that would indeed be a prime example of using the term as a catch-all despite that it's been well understood in the literature for a long time now that the two are separate effects. It can be tricky semantically, but I think it's best to use the terms more rigorously.

And yeah, the effects are very much real! It's been observed since way back too, and you can see the same as in that chart at pretty much any time. See e.g. the same from around 1950-1970 instead, and the same variation is clearly visible, year in and year out with few exceptions:

Edited March 2Mar 2 by Philalethes
RTSW plot

@Philalethes Sorry, I did not write it correctly. I meant to say that, ignoring the 10 deg deviation of the dipole model from the geographic pole, the z axis coincides with the dipole axis at equinox. It must be because the rotational axis is perpendicular to x at equinoxes and it deviates from z the most at solstices. Thus, at equinox Bx or By have no effect on the projection of the IMF vector on the dipole axis. This thread started with trying to explain why we can see aurora when Bz > 0 @Samrau's hypothesis was By can save the day. So, this hypothesis is incorrect because at equinox By or Bx have no effect on the projection of the IMF on the dipole axis, right?

On the other hand, at solstices Bx plays a role on this projection. In June, we would want Bx < 0 and in December Bx > 0. I can see this from the geometry. Why is this not a thing then?

I get why due to the bending we have in a typical solar wind By > 0 & Bx < 0 (outward) or By < 0 & Bx > 0 (inward). What I still do not see is why By < 0 implies (higher chances of) Bz < 0. These are independent components of a vector, thus why should Bz depend on the sign of By?

1 minute ago, JessicaF said:

Sorry, I did not write it correctly. I meant to say that, ignoring the 10 deg deviation of the dipole model from the geographic pole, the z axis coincides with the dipole axis at equinox. It must be because the rotational axis is perpendicular to x at equinoxes and it deviates from z the most at solstices.

Yep, that's all exactly right. At the equinoctes, if we disregard the deviation of the dipole from the geographic pole, then there's already zero component in the x-axis, so the dipole is then in the yz-plane already. Twice a day that will actually be true even if you do include that ~10° deviation (in fact, it will be true twice a day no matter the time of the year too in that case).

5 minutes ago, JessicaF said:

Thus, at equinox Bx or By have no effect on the projection of the IMF vector on the dipole axis.

Well, they never have any such effect, at least not in this context; they're just associated with a direction of field which happens to have a higher or lower likelihood of a positive or negative Bz depending on polarity and which equinox it is. When you've converted to GSM coordinates and are operating with Bx, By, and Bz, then the projection has already happened, and at that point only the Bz (and to some extent the By, as per the coupling function, but that's a separate matter) is relevant.

8 minutes ago, JessicaF said:

This thread started with trying to explain why we can see aurora when Bz > 0 @Samrau's hypothesis was By can save the day. So, this hypothesis is incorrect because at equinox By or Bx have no effect on the projection of the IMF on the dipole axis, right?

I believe you're referring to the other thread; but the questions you're posing here belong in this thread anyway. The By can save the day as per the physics of the coupling between the solar wind and the magnetosphere (the Bx cannot), as is the subject of the other thread, but that's not the kind of projection effect that the R-M effect is about. The two are very separate things, and should not really be mixed; hence the two separate topics, heh.

11 minutes ago, JessicaF said:

On the other hand, at solstices Bx plays a role on this projection. In June, we would want Bx < 0 and in December Bx > 0. I can see this from the geometry. Why is this not a thing then?

That is a great question; the reason for that is because the solar wind flows radially outward, which results in only the components in the yz-plane being relevant to the effects. Otherwise you would have been correct, and you'd essentially have R-M peaks when both components were aligned with the actual direction of the planet's tilt (so you'd get peaks sometime between the solstices and the equinoctes instead). Talking about the Bx and By separately in that context is actually no point in any case, because they go together due to the outwards or inwards direction of the field and the Parker spiral, hence the RTSW plot I posted above (I edited it in, so you might have to refresh again if you didn't see it) where you can see that the phi-angle has a strong tendency to ~135° (positive By, negative Bx, "away") and ~315° (negative By, positive Bx, "towards"). These are ~45° off the radial and tangential directions, so in other words the Parker spiral bends the field just enough for the two components to match each other in magnitude.

But yeah, since only the yz-plane matters the Bx is simply irrelevant for pretty much everything, heh. The GSM projects the dipole to the yz-plane, so at the solstices you essentially just get a z-axis that matches that of GSE coordinates. This is not conducive in terms of the R-M effect, because the field at that point won't be leaning against the tilt in either direction, since it's perpendicular to it.

Thanks for all this info. You probably missed my last question above since I edited it later. @Philalethes

I get why due to the bending we have in a typical solar wind By > 0 & Bx < 0 (outward) or By < 0 & Bx > 0 (inward). What I still do not see is why By < 0 implies (higher chances of) Bz < 0. These are independent components of a vector, thus why should Bz depend on the sign of By?

Edited March 2Mar 2 by JessicaF

Just now, JessicaF said:

Thanks for all this info. You probably missed my last question above since I edited it later.

I get why due to the bending we have in a typical solar wind By > 0 & Bx < 0 (outward) or By < 0 & Bx > 0 (inward). What I still do not see is why By < 0 implies (higher chances of) Bz < 0. These are independent components of a vector, thus why should Bz depend on the sign of By?

Yeah, I missed that.

The thing is: they are not independent after all! When the field is in either direction it's a result of the field as it emanates from the solar surface, and the components thus imply something about each other. For the most basic hypothetical case we can simply think of the Earth and sun as having zero axial tilt and zero pesky complexity, and thus the field lines would all lie directly in the ecliptic plane in our neat scenario. In GSE coordinates, the magnetic field hitting Earth would have zero z-component, and only x- and y-components. But what happens if you slightly tilt Earth? Well, in GSE coordinates the same remains true, since they're defined with respect to the orbit, but if you now transform it to the GSM coordinates of this new tilt you see that now you suddenly lose a little bit of the y-component and gain a little bit of z-component; but the y-component will still largely be in a given direction depending on the field. That's the reason they're not independent, and to the extreme, if you tilted Earth 90° sideways, now suddenly the entire y-component would be gone, replaced by a z-component. Or, if you wanted to be a bit less extreme, add a little bit of axial tilt to the sun instead (which it indeed has), and now you have to consider the equatorial axis of the sun as well to determine the direction (which is why the R-M effect peaks a couple of weeks after the equinoctes rather than right on them).

16 minutes ago, Philalethes said:

Yeah, I missed that.

The thing is: they are not independent after all! When the field is in either direction it's a result of the field as it emanates from the solar surface, and the components thus imply something about each other. For the most basic hypothetical case we can simply think of the Earth and sun as having zero axial tilt and zero pesky complexity, and thus the field lines would all lie directly in the ecliptic plane in our neat scenario. In GSE coordinates, the magnetic field hitting Earth would have zero z-component, and only x- and y-components. But what happens if you slightly tilt Earth? Well, in GSE coordinates the same remains true, since they're defined with respect to the orbit, but if you now transform it to the GSM coordinates of this new tilt you see that now you suddenly lose a little bit of the y-component and gain a little bit of z-component; but the y-component will still largely be in a given direction depending on the field. That's the reason they're not independent, and to the extreme, if you tilted Earth 90° sideways, now suddenly the entire y-component would be gone, replaced by a z-component. Or, if you wanted to be a bit less extreme, add a little bit of axial tilt to the sun instead (which it indeed has), and now you have to consider the equatorial axis of the sun as well to determine the direction (which is why the R-M effect peaks a couple of weeks after the equinoctes rather than right on them).

GOTCHA! As you said, there is a huge difference between a regular SW magnetic vector and one inside a CME. A regular SW basically transfers the polarity from the surface, hence we get "prevailing" wind directions and thus dependencies. This cannot be said about the mess called CME. So, these spring days we root for inward (negative) CHs and vice versa in the fall. Looking at the Bz from the past two days, it seems that we were (and still are) under the influence of a + polarity CH, right? Can one tell before the HSS comes what polarity it will be? If it is going to be a good or a bad CH?

1 minute ago, JessicaF said:

GOTCHA! As you said, there is a huge difference between a regular SW magnetic vector and one inside a CME. A regular SW basically transfers the polarity from the surface, hence we get "prevailing" wind directions and thus dependencies. This cannot be said about the mess called CME.

Yep, exactly correct. CMEs have their own mini-versions of the same kind of phenomenon in the form the orderly flux ropes, where you can often tell that a certain component is about to sweep across a certain range (and if that is Bz going negative, like for ESW and WSE ropes, then aurora hunters should all be out the door with camera in hand). But during the turbulent sheaths of the CMEs, as well as during the turbulent SIRs of CHs, then there's generally just no prevailing direction whatsoever.

5 minutes ago, JessicaF said:

So, these spring days we root for inward (negative) CHs and vice versa in the fall.

Indeed!

6 minutes ago, JessicaF said:

Looking at the Bz from the past two days, it seems that we were (and still are) under the influence of a + polarity CH, right? Can one tell before the HSS comes what polarity it will be? If it is going to be a good or a bad CH?

Correct. And yes, you can tell the polarity of the CHs from the synoptic maps, so you know well in advance (unless the CHs form out of nowhere very fast) which polarity you're dealing with. If you e.g. look at the synoptic map I posted above, you can see this CH being a positive one.

Extremely useful discussion, @Philalethes . Thank you so much for everything! Plus, let's finish that little project on adding Newell's coupling to the SWL!

I included the link to the synoptic maps to my bookmarks. This should be a must for any aurora hunter :)

I award the honorable @Philalethes the title of Philosophy Doctor 🧑‍🎓

13 hours ago, JessicaF said:

@Philalethes Sorry, I did not write it correctly. I meant to say that, ignoring the 10 deg deviation of the dipole model from the geographic pole, the z axis coincides with the dipole axis at equinox. It must be because the rotational axis is perpendicular to x at equinoxes and it deviates from z the most at solstices. Thus, at equinox Bx or By have no effect on the projection of the IMF vector on the dipole axis. This thread started with trying to explain why we can see aurora when Bz > 0 @Samrau's hypothesis was By can save the day. So, this hypothesis is incorrect because at equinox By or Bx have no effect on the projection of the IMF on the dipole axis, right?

It's not my hepotize, I didn't write about it. I don't even understand what you and @Philalethes hes wrote here, it would take me years to figure it out.

ha-ha

5 minutes ago, Samrau said:

It's not my hepotize, I didn't write about it. I don't even understand what you and @Philalethes hes wrote here, it would take me years to figure it out.

ha-ha

You were right, I should edit the posting above. Strong By + high enough wind speed can combine to cause a strom even when Bz is about zero.