It would have been from the CH, but noting if it falls under the category of a SIR the entry would have been around /19 18:40Z

There seems to be a fair amount of aurora sightings now; I saw people say they saw it in Netherlands and Norfolk, both around 50° N. I think this might be a situation where what @Bry posted not long ago could be of particular interest. One of the most interesting effects pointed out in the paper (this one) is how the polarity of the IMF is quite relevant to when in the year geomagnetic activity is more likely, an asymmetry explained by the effect it focuses on (the Russell-McPherron effect). I'd come across this when I investigated why there seemed to be more activity near the equinoctes, but that it also causes more activity in the earlier part of the year (like right now) when the IMF polarity is negative and vice versa was something I didn't know; and it just so happens that the IMF polarity just turned negative. Apparently there's a very clear and measurable increase in the chance of a negative Bz during this part of the year when that happens. The image that was posted previously was with regards to the Dst index, whereas the corresponding plot for the AE index looks like this (black line is roughly the day of the year we're at now):

So interestingly the direction of By whenever we're not talking about a CME coming through (i.e. Solar wind and presumably also effects from CHs) can be used as a quick indicator to look at for whether or not there's likely to be a negative Bz. This might help explain why we're seeing that now, and since we'll still be in the negative polarity for the larger CH that's about to pass us we might perhaps see more activity then too. It certainly seems to be reflected in the AE-index currently:

I've seen someone suggest that this might have been some stealth CME too, but I'm not sure how well that fits. Seems to potentially be explained in this way at least, but it could always be wrong. Would certainly like to know what others think.

It seems that it reached the geomagnetic storm on a scale of g2

Aaaaand Bz just shifted North.

11 minutes ago, cgrant26 said:

Aaaaand Bz just shifted North.

Should go back south in a bit, although not quite as strong as before

According to STEREO data

The velocities and temps are up and density is down, so the HSS kicked in.

 

33 minutes ago, Sam Warfel said:

According to STEREO data

Not that STEREO A is out in a different position, but the coordinate systems used are different. STEREO has RTN components and I believe L1 uses GSE components x, y, and z. BR would match Bx (Satellite-Sun line and Earth-Sun line), but it is a little different for the other 2 components.

Edited May 20, 20232 yr by Jesterface23

10 hours ago, Philalethes said:

One of the most interesting effects pointed out in the paper (this one) is how the polarity of the IMF is quite relevant to when in the year geomagnetic activity is more likely, an asymmetry explained by the effect it focuses on (the Russell-McPherron effect). I'd come across this when I investigated why there seemed to be more activity near the equinoctes, but that it also causes more activity in the earlier part of the year (like right now) when the IMF polarity is negative and vice versa was something I didn't know; and it just so happens that the IMF polarity just turned negative. Apparently there's a very clear and measurable increase in the chance of a negative Bz during this part of the year when that happens.

From what I understand (but I may be wrong, if anyone knows better please correct me) this is related to the tilt between the ecliptic plane and the Sun's axis of rotation, which amounts to ~7°. While the heliospheric current sheet (where the polarity flips) is not flat but twisted, its average plane is aligned with the Sun. The right ascension of Sun's axis of rotation is 286.13°; this is how far eastwards from the spring equinox the two planes intersect; since this is close to a right angle this means the distance between the two planes is at the largest, and consequently the average expected B_z is furthest away from zero too right around the equinoctes.

6 hours ago, noordung said:

From what I understand (but I may be wrong, if anyone knows better please correct me) this is related to the tilt between the ecliptic plane and the Sun's axis of rotation, which amounts to ~7°. While the heliospheric current sheet (where the polarity flips) is not flat but twisted, its average plane is aligned with the Sun. The right ascension of Sun's axis of rotation is 286.13°; this is how far eastwards from the spring equinox the two planes intersect; since this is close to a right angle this means the distance between the two planes is at the largest, and consequently the average expected B_z is furthest away from zero too right around the equinoctes.

As far as I understand it's indeed partially due to the angle of the heliomagnetic field determined by the Solar axis of rotation, but I'm not quite sure the last part is correct, although it might be that I just don't understand exactly what you're saying. At least I would note that right ascension is in terms of the equatorial coordinate system and not the ecliptic, so I'm not sure exactly which plane you're talking about there. Since the rotational axis of Earth itself is tilted with respect to the ecliptic, the Solar equatorial plane intersects the ecliptic plane at different line than where it intersects Earth's equatorial plane; with respect the the ecliptic, the Solar rotational axis points towards ~346° of ecliptic longitude, i.e. directly away from us and towards us ~14 days before the March and September equinox respectively (0° being the March equinox in both coordinate systems). I would guess you know this by the sound of it, so I would guess I might've misinterpreted something.

To clarify in any case, the Russell-McPherron effect primarily concerns itself with the angle between a plane parallel to the Solar equatorial plane and the axis of Earth's geomagnetic dipole projected onto a plane perpendicular to the X-axis (which is the line straight from Earth to Sol, in the ecliptic plane). As is explained in the introduction of the paper:

Quote

[...] the R-M effect holds that the angle between Z axis in geocentric solar magnetospheric (GSM) coordinate system and Y axis in geocentric solar equatorial (GSEQ) coordinate system plays an important role. Figure 1a shows the semiannual and diurnal variation of the angle θ between the Z axis in GSM coordinate system and the Y axis in GSEQ coordinate system, that is, the controlling parameter of the R-M effect. According to the R-M effect, the probability of southward IMF increases when the angle θ, which is smaller than 90 degrees, decreases, so that the dayside reconnection can be more efficient and more energy can be conveyed into the magnetosphere.

The relation between those axes in those coordinate systems is what I tried to convey above. In simpler terms, one can think of the former plane being 0° ~2 weeks before each equinox, and ~7.25° to either side ~2 weeks before each solstice, while the latter plane more or less being defined by Earth's axial tilt, with the Z-component being ~23.5° to either side at the equinoctes, and 0° at the solstices, with a diurnal variation of ~10° to either side in both cases due to Earth's geomagnetic dipole being offset by ~10° from its rotational axis. Thus one would expect the minimum such angle to be near the equinoctes, during the time of the day when the geomagnetic dipole tilt adds to Earth's axial tilt for a total of ~23.5° + ~10° = ~33.5°, causing the angle to be as little as 90° - ~33.5° = ~56.5°; indeed by looking at figure 1a, that's what seems to be the case:

So perhaps the most important thing to note is that what's considered important for the effect is the alignment of only the Z-component of Earth's geomagnetic dipole with the Bz of the IMF as determined by the heliomagnetic field. Maybe you already got that; feel free to clarify or point it out if I misinterpreted what you said.

It seems that there is an effect of CH, right now 10 bt and 10 bz to the south

well geomagnetic storm kp6 arrived

Timing is everything for us earthbound kids, as well as weather,  bz etc.  Waiting on that x class massive CME can be frustrating at times! Ha! 

and the bz continues to the south I wonder how long the effects of CH last

There's different CHs back to back to back, and the big one hasn't even arrived yet.

On 5/19/2023 at 6:47 PM, Philalethes said:

There seems to be a fair amount of aurora sightings now; I saw people say they saw it in Netherlands and Norfolk, both around 50° N. I think this might be a situation where what @Bry posted not long ago could be of particular interest. One of the most interesting effects pointed out in the paper (this one) is how the polarity of the IMF is quite relevant to when in the year geomagnetic activity is more likely, an asymmetry explained by the effect it focuses on (the Russell-McPherron effect). I'd come across this when I investigated why there seemed to be more activity near the equinoctes, but that it also causes more activity in the earlier part of the year (like right now) when the IMF polarity is negative and vice versa was something I didn't know; and it just so happens that the IMF polarity just turned negative. Apparently there's a very clear and measurable increase in the chance of a negative Bz during this part of the year when that happens. The image that was posted previously was with regards to the Dst index, whereas the corresponding plot for the AE index looks like this (black line is roughly the day of the year we're at now):

So interestingly the direction of By whenever we're not talking about a CME coming through (i.e. Solar wind and presumably also effects from CHs) can be used as a quick indicator to look at for whether or not there's likely to be a negative Bz. This might help explain why we're seeing that now, and since we'll still be in the negative polarity for the larger CH that's about to pass us we might perhaps see more activity then too. It certainly seems to be reflected in the AE-index currently:

I've seen someone suggest that this might have been some stealth CME too, but I'm not sure how well that fits. Seems to potentially be explained in this way at least, but it could always be wrong. Would certainly like to know what others think.

 

 

On 5/12/2023 at 1:01 PM, Bry said:

This article suggests that -Bz geomagnetic storms are more likely during the spring equinox when the jet stream crosses through the northern hemisphere due to earths tilt (or Russel-McPherron effect) at that time.

“[2] The semiannual variation in geomagnetic activity has been recognized for a long period of time [Cortie, 1912], which shows the maximum appears around equinoxes while the minimum appears around solstices, e.g., geomagnetic storm annual distribution [Echer et al., 2011]. Over the decades, several explanations for this variation have been put forward, such as the axial hypothesis, the equinoctial hypothesis and the Russell-McPherron effect [Cortie, 1912; Bartels, 1932; McIntosh, 1959; Svalgaard, 1977; Russell and McPherron, 1973].”

I wonder if this is just for the northern hemisphere and if people in the Southern Hemisphere might have a different experience.

I would expect future CME’s to have a more +Bz or positive/northern magnetic field component as we exit the spring equinox aurora season based off this data. I’m wondering if experienced aurora hunters find this to be generally true.

I don’t see auroras at my latitude so I can’t confirm either way but I see the importance of being able to communicate the diversity in people’s experiences between the Northern vs Southern Hemisphere considering the potential impacts of the South Atlantic Anomaly.

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2012JA017845#:~:text=Geomagnetic activity are rather strong,the Sun%2C which is identical

 

Seems like @Philalethes found the same article I originally posted about:

-Bz geomagnetic storms being more prevalent during spring equinox and +Bz geomagnetic storms being more prevalent during fall equinox during solar max due to earths rotational and orbital tilt at that time.

Glad this is being discussed, because I  actually have a ton of burning comments and questions about it. 

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

1. If -Bz storms are more likely during spring equinox, what does that mean when there are -Bz geomagnetic storms during the fall equinox or other times of the year?

 I noticed one of the biggest geomagnetic storms in the archive was that 2003 Halloween storm which had a rather negative or southward Bz magnetic field component when the US was at war in Iraq. Is it possible that auroras visible in the fall could be man-made?

2. Should we not expect any more -Bz storms or visible auroras at all on earth until next spring or just in the northern hemisphere?

3. Are auroras more visible in the Southern Hemisphere near the equator & South Atlantic anomaly certain times a year based off of this data?

(I’m sorry to see anyone paying attention to space weather from the equator like the Colombian participating on the forum leave from lack of adequate spam or Spanish translation.. 

Its funny I lived with a Brazilian all last summer and had loads of cultural/translational differences especially when he started filming us w/o asking and I remember him saying we were not appreciative of his confident or “conquistador-like” culture...lol.. so I assume translational errors are usually the heart of problems and hope those confident enough to communicate aren’t discriminated for it is all.)

VLF

I just recently learned our VLF communication system for nuclear submarines since the 1960’s has created a man-made bubble around earth that can block solar flares and CMEs (see article links below)...

https://spaceweatherarchive.com/2017/05/19/anthropogenic-space-weather/#:~:text=Other anthropogenic effects on space,formed by chemical release experiments.

https://arxiv.org/pdf/1611.03390.pdf

https://www.theatlantic.com/science/archive/2017/05/wow-guys/527193/

https://www.nasa.gov/feature/goddard/2017/nasas-van-allen-probes-spot-man-made-barrier-shrouding-earth

 

My question regarding their effect on geomagnetic activity is:

4. - How might this VLF “bubble” affect or interact with solar flares or geomagnetic storms or our predictions considering it has been around since the 1960s? 

5. & does this explain why the strongest solar cycle (SC19) was right before we started using VLF to communicate? 

6. does this mean all future solar cycles  can be expected to be weaker than we have seen in the past since before VLF communications in the 1960s, or only when we use them?

Apparently this very low frequency bubble is visible from space and keeps the Van allen radiation belt particles farther from earth than would happen naturally.

”The probes have noticed an interesting coincidence — the outward extent of the VLF bubble corresponds almost exactly to the inner edge of the Van Allen radiation belts, a layer of charged particles held in place by Earth’s magnetic fields. Dan Baker, director of the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder, coined this lower limit the “impenetrable barrier” and speculates that if there were no human VLF transmissions, the boundary would likely stretch closer to Earth. Indeed, comparisons of the modern extent of the radiation belts from Van Allen Probe data show the inner boundary to be much farther away than its recorded position in satellite data from the 1960s, when VLF transmissions were more limited.”

Sorry for the long post, & hope my questions don’t dampen any aurora season prospects.

 

Edited May 25, 20232 yr by Bry
Added two better articles on VLF

12 hours ago, Bry said:

Seems like @Philalethes found the same article I originally posted about

Well, it wasn't that I somehow found it by coincidence of course, I was directly referencing your post since I read into it a bit after you first posted it, it just seemed to be particularly relevant to the geomagnetic activity we experienced when I wrote that, which is why I thought I'd mention it. It's definitely something every aurora hunter should take note of, and the article is very informative in that regard.

12 hours ago, Bry said:

-Bz geomagnetic storms being more prevalent during spring equinox and +Bz geomagnetic storms being more prevalent during fall equinox during solar max due to earths rotational and orbital tilt at that time.

That's not quite what it says; it's still a negative Bz that leads to higher levels of geomagnetic activity in both cases, the interesting part is how each polarity of the By is associated with a negative Bz in its own part of the year, i.e. a negative By will tend to have a negative Bz in the earlier part of the year, centered around the March equinox, while a positive By will tend to have a negative Bz in the later part of the year, centered around the September equinox. That is essentially the heart of the R-M effect. Thus anyone preparing for an aurora hunt around those times (but also other parts of the year to varying extents) should definitely be taking note of the IMF polarity and account for it accordingly.

It doesn't have to do with Solar maximum as far as I'm aware, and it should apply during minimum too; definitely relevant for when there are more coronal holes both in general and at the equator near minimum, even if geomagnetic activity tends to be lower overall.

12 hours ago, Bry said:

1. If -Bz storms are more likely during spring equinox, what does that mean when there are -Bz geomagnetic storms during the fall equinox or other times of the year?

2. Should we not expect any more -Bz storms or visible auroras at all on earth until next spring or just in the northern hemisphere?

The above should hopefully answer both those question, as it's still a negative Bz that tends to lead to geomagnetic activity in both cases; it's rather about how different By polarities lead to a more negative Bz in different parts of the year. We should definitely be expecting geomagnetic activity this fall in both hemispheres; but instead of now, when we should be more on the lookout for a negative By (although it's less and less relevant the closer we get to the solstice), we should then be more aware of when the By is positive instead. Since the IMF doesn't flip that often, it should be noted and accounted for when hunting for aurorae, since one direction will predominate (phi angle of ~90°-180° when the IMF is positive, i.e. away, and a phi angle of ~270°-360° when the IMF is negative, i.e. towards).

12 hours ago, Bry said:

3. Are auroras more visible in the Southern Hemisphere near the equator & South Atlantic anomaly certain times a year based off of this data?

To the best of my knowledge that's not the case, at least not actual aurorae as we think of them, and there's nothing inherently about the R-M effect that is preferential to either hemisphere on Earth, so if there's increased activity in either part of the year it'll register in both hemispheres. As for the geomagnetic field being noticeably weaker around the SAA it's still more than strong enough to prevent any direct auroral activity; but interestingly there are papers like this one describing how there are increased levels of "aurora-like" airglow there due to more particles reaching the upper atmosphere:

Quote

An area where the geomagnetic field differs significantly from the expected symmetric dipole, such as at the South Atlantic Anomaly, where the magnetic field intensity is low, gives rise to stronger precipitation of energetic particles into the upper atmosphere. Impact excitation and the subsequent airglow emissions exhibit aurora-like dynamic signatures.

So while it's not quite the aurora as we think of it, since that requires particles being led right into the atmosphere via the field, it would rather be an increased level of particles getting deeper into the magnetosphere there. From what I've read it's estimated that the inner Van Allen belt can dip down to ~200 km above the surface near the SAA. It's certainly not going to match any proper aurorae anytime soon, but it's definitely interesting to note.

12 hours ago, Bry said:

4. - How might this VLF “bubble” affect or interact with solar flares or geomagnetic storms or our predictions considering it has been around since the 1960s?

5. & does this explain why the strongest solar cycle (SC19) was right before we started using VLF to communicate? 

6. does this mean all future solar cycles  can be expected to be weaker than we have seen in the past since before VLF communications in the 1960s, or only when we use them?

The topic of VLF and its effects is certainly an interesting one, although a somewhat different subject. I've also come across this at various points, but it's not entirely straightforward to separate out what the actual facts are or what desirable or undesirable consequences might come from it in terms of geomagnetic activity or otherwise. It's probably better to take discussion about that in particular over to the Unproven theories thread; it could certainly be relevant to geomagnetic activity, and there are many indications that it is, but it's not so clear what the exact relationship is.

Edited May 25, 20232 yr by Philalethes
typo

Does density typically need to be above a certain threshold in conjunction with negative Bz before one could expect aurora in the northern upper midwest?     Said more generially, what role does density play in auroras?

10 hours ago, Strom said:

Does density typically need to be above a certain threshold in conjunction with negative Bz before one could expect aurora in the northern upper midwest?     Said more generially, what role does density play in auroras?

As far as I'm aware there's not inherently any threshold for density alone, but the combination of density and speed must typically reach a certain level before geomagnetic activity becomes noticeable. There's a quantity which isn't shown on the site known as flow pressure (corresponding more or less to dynamic pressure in fluid dynamics) which is calculated from the two (as well as some other factors, but these are typically very close to constant). The relationship is directly proportional to the density and proportional to the square of the speed, so one could say that speed is arguably more important than density in that regard.

So in relatively simplified terms, what you're looking for is a combination of sufficient flow pressure and a negative Bz. Here's a quick plot of the variables in question so far this year, i.e. the Bz, the flow pressure, and the ap60-index (a measure of hourly geomagnetic activity); also added in on the third plot is a product of flow pressure and -Bz to see the relationship more clearly:

Here it should be readily seen that whenever flow pressure * -Bz goes up, there's usually an increase in geomagnetic activity.

Philalethes, thank you for sharing your knowledge and expertise.   Very interesting graphs of the data.  In some respect, my first impression is that flow pressure sounds very similar to kinetic energy (KE=1/2mV^2).  Perhaps higher kinetic energy particles results in a bigger impact as they enter, resulting in more intense auroras, given the right Bz condition.

Edited May 28, 20232 yr by Strom

1 hour ago, Strom said:

Philalethes, thank you for sharing your knowledge and expertise. Very interesting graphs of the data.  In some respect, my first impression is that flow pressure sounds very similar to kinetic energy (KE=1/2mV^2).  Perhaps higher kinetic energy particles results in a bigger impact as they enter, resulting in more intense auroras.

Yep, that's pretty much exactly analogous indeed as far as I know. If you look at e.g. the Wikipedia article on dynamic pressure, you'll see that it's the same formula only with the differing quantities: q = 1/2 * ρ * u^2, where ρ (rho) is the density and u is the speed of the flow. It even states the analogy explicitly:

Quote

It can be thought of as the fluid's kinetic energy per unit volume.

Ultimately the aurorae are caused by particles gradually imparting their kinetic energy to the atmosphere as they bounce back and forth along the magnetic field, so it's intuitively sensible that this would be related to the level of geomagnetic activity. The more energetic they are the deeper they penetrate too, both in terms of how close to the surface they get before bouncing and how much time they spend inside the atmosphere due to entering it at lower latitudes at a more oblique angle.

If you want to calculate the flow pressure of the live Solar wind, you can easily plug the values into the formula used to derive it, described on the OMNI website here (which is where the Bz and flow pressure plotted above is from). It's simply the above formula times the factor they state that you can reasonably assume to be a constant, 1.2 in this case. Plugging in current values into e.g. WolframAlpha (0.47 p/cm^3 and 425 km/s when I just checked SWL) we get a flow pressure of ~0.17 nanopascals (nPa, the unit of the flow pressure plot that I forgot to include in the axis label). Judging by the above plot it looks like you typically want a relatively sustained value of around 7.5 nPa or more combined with a sustained negative Bz for the heftiest geomagnetic storms.

Edited May 28, 20232 yr by Philalethes
corrected values in the WA link

Gave some thought to the information above.  So if a 7.5 nPa was set as threshold to reach a hefty geomagnetic storm (with a sustained -Bz), then there would be a combination of density and speed that could reach that threshold.  Here is a graph I worked up; a point on or above the graph would result in a aNp at or above 7.5

Edited May 28, 20232 yr by Strom

There is then which satellite best represents the graph. Typically ACE has the lower density values, while DSCOVR is higher and SOHO is somewhere in the middle. ACE's and DSCOVR's velocities are fairly similar, while SOHO's velocities are a little higher.

2 hours ago, Strom said:

Gave some thought to the information above.  So if a 7.5 nPa was set as threshold to reach a hefty geomagnetic storm (with a sustained -Bz), then there would be a combination of density and speed that could reach that threshold.  Here is a graph I worked up; a point on or above the graph would result in a aNp at or above 7.5

Yeah, that plot would be it for that flow pressure indeed; good visual aid. That being said I was just eyeballing that tick on the plot, so it's probably not always around there; and the threshold of what's "hefty" isn't exactly well-defined either, although I'd say we'd be talking about G3 or above, maybe some would even call a G2 that.

Looking over the same plot at different time periods I notice that the flow pressure typically ends up a lot higher during the brunt of the largest storms, although activity definitely tends to pick up already at those levels. Rummaging around for the opposite, where you get a good amount of activity from flow pressures not that high, I did find some, like the storm of September 7-8, 2017, which peaked twice at G4 and was relatively sustained at G3 for a few hours near those peaks; there the flow pressure didn't even reach 8 during the hours of most activity, and lay at a 6-hour average of 2-6 for the entirety of it. Here I've tried to zoom in to show the relevant period (these are all 6-hour averages):

As you can tell, the first part of the storm hit when the flow pressure was just ~2-3, clearly owing to the sustained strongly negative Bz for those hours. I also kept in the part of the plot on the left just to show how the higher flow pressure before that didn't have much of an effect at all due to the positive Bz.

I was going to contrast that with the 2003 Halloween storms, but interestingly the flow rate data from those days is lacking, presumably because it overpowered the satellites or something of that sort. Instead I chose the storm of March 31, 2001, as it seemed to be the most energetic at a glance and actually had all the values in the data:

Here you can see what I meant about the flow pressure, which reaches sustained levels of ~20-25 for several hours, and that's still the 6-hour average (peaking as high as 44 for a single hour in the data). Interestingly, here the activity even starts going up while the Bz is still positive, perhaps owing precisely due to the rapidly increasing flow pressure.

So all in all, while the largest storms tend to have very high flow pressures, there are clearly relatively smaller ones that are still quite strong which occur with lower ones, so with the right Bz you don't need to get as high as 7.5 before you start seeing activity. Here's the one from last month (April 23-24, 2023) for good measure:

With this view we can see that here too geomagnetic activity starts to pick up already when the flow pressure is still around 2-6, and the second peak of activity also came when the pressure was around that range, in both cases primarily due to the strongly negative Bz. Seems to be around a flow pressure * -Bz of around 20-40 that activity usually starts increasing the most (again some eyeballing on my part, it's always possible to look even closer); I'm not sure that's really a linear relationship though, I would suspect that it's not, so it's more to give some idea.

22 minutes ago, Jesterface23 said:

There is then which satellite best represents the graph. Typically ACE has the lower density values, while DSCOVR is higher and SOHO is somewhere in the middle. ACE's and DSCOVR's velocities are fairly similar, while SOHO's velocities are a little higher.

Definitely a good point. The above data is the hourly averages from OMNI, which uses some form of comparison between them all; as they describe:

Quote

The Low Resolution OMNI (LRO) data set is primarily a 1963-to-current compilation of hourly-averaged, near-Earth solar wind magnetic field and plasma parameter data from several spacecraft in geocentric or L1 (Lagrange point) orbits. The data have been extensively cross compared, and, for some spacecraft and parameters, cross-normalized. Time-shifts of higher resolution data to expected magnetosphere-arrival times are done for data from spacecraft in L1 orbits (ISEE 3, Wind, ACE), prior to taking hourly averages.

So to use any live data you would surely need some idea how the satellite from which the data originates typically compares to that dataset; I would guess getting some idea of how ACE and DSCOVR in particular compare to it would be ideal for now, since they're the ones responsible for the most looked-at RTSW measurements.

Edited May 29, 20232 yr by Philalethes
grammar

On 5/28/2023 at 6:04 PM, Strom said:

Gave some thought to the information above.  So if a 7.5 nPa was set as threshold to reach a hefty geomagnetic storm (with a sustained -Bz), then there would be a combination of density and speed that could reach that threshold.  Here is a graph I worked up; a point on or above the graph would result in a aNp at or above 7.5

Nice graph!  I had thought a lot about this awhile ago, and had looked into coupling functions that try to estimate the auroral energy as a function of these parameters.  A nice overview of a lot of these functions can be found here: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2021JA029946

Many of these functions, empirical or otherwise have the 1/2*rho*v**2 embedded within them one way or another.  Similar to your graph, I had toyed around a little bit trying to determine a phase-space based on these functions that could give some indications of auroral activity, but it's a little difficult to visualize (see attached image, that shows somewhat recent values plotted on the phase-space).  At the very least, the contours have roughly the same shape as your chart and clearly show the relationship between density and speed.

Anyway, cool discussion to have while the sun seems to be taking a break from its flaring activity, and while the NH aurora season mostly wraps up.

Contoured is the auroral power in GW, and the scatter plot color shows the Bz.

 

Tamitha Skov's latest Solar Storm Forecast just came out. She's thinking the Western directed CME from the 4th may give us a glancing blow due to the higher solar wind speed from the CH that's been facing us the last week or so. The prediction is mid day today through tomorrow for high to maybe mid latitude aurora. Fingers crossed!

 

 

2 hours ago, cgrant26 said:

Tamitha Skov's latest Solar Storm Forecast just came out. She's thinking the Western directed CME from the 4th may give us a glancing blow due to the higher solar wind speed from the CH that's been facing us the last week or so. The prediction is mid day today through tomorrow for high to maybe mid latitude aurora. Fingers crossed!

I'm not sure I entirely agree with her reasoning about the fast Solar wind. I've mentioned Bernoulli's principle before in similar contexts, and while there probably are some reasons why it doesn't apply perfectly to Solar wind (e.g. being somewhat compressible and electromagnetic effects) I believe the main principle still stands, i.e. that an increase in speed leads to a decrease in static pressure (the pressure exerted perpendicular to the direction the fluid is flowing); and in fact, that the Solar wind is indeed somewhat compressible and that the fast wind has a lower density than the slow one should, if I'm not entirely mistaken, only serve to exacerbate that effect.

So in other words, as far as I can tell the fast wind will do the opposite in that position, and actually lead to less deflection of the CME than if it were plain old slow wind. I do however agree that Solar wind in general will indeed tend to deflect CMEs in that direction if the CME is fast enough (presumably due to similar reasons as above, i.e. that this would mean less static pressure on the Solar wind beside it), but for slow ones the opposite can be the case. Here's a paper discussing this to some extent; quoting from the abstract:

Quote

Under the effect of the Parker spiral magnetic field, a fast CME will be blocked by the background solar wind ahead and deflected to the east, whereas a slow CME will be pushed by the following background solar wind and deflected to the west. The deflection angle may be estimated according to the CMEs'' transit speed by using a kinetic model. It is shown that slow CMEs can be deflected more easily than fast ones. This is consistent with the observational results obtained by Zhang etal. (2003), that all four Earth-encountered limb CMEs originated from the east. On the other hand, since the most of the EFHCMEs are fast events, the range of the longitude distribution given by the theoretical model is E40,W70, which is well consistent with the observational results (E40,W75).

Given that this did not appear to be a halo CME and didn't seem that fast, I'd say that makes it less likely, and if what I've mentioned so far about the change in pressure is in fact true then you'd expect that to lead to it being even less likely.

But I could certainly be wrong (maybe there's something about fluid dynamics in general or the Solar wind in particular I'm missing), and/or the CME could end up hitting us anyway. Just sharing what I think based on my understanding of a simple model of the physics involved.

Edited June 7, 20232 yr by Philalethes
typo