Hello,

I don't know if my question is right here.

Also, please bear with me, if it is a stupid one.

I haven't been involved in speace weather for too long.

I keep reading over and over again that a negative BZ is necessary for the development of the aurora. I also understand the corresponding models and their explanations.

Now I have seen all of Tamitha Skov's courses and in her course "All about indices" she says that even a positive BZ with a negative BY can lead to aurora.

Michael Theusner also says this. In a WhatsApp group about spotting the aurora, a lady also reported seeing them with a positive BZ in northern germany. Some of the group said this was impossible.

I am confused.

Many models only assume a negative BZ and the BY is not discussed. 

Can someone explain to me what processes lead to auroras being visible even with positive BZ and negative BY? 

Or are there models that explain this clearly?

Please excuse me if it's a stupid question 🙈 and also my spotty English. It's not my native language.

If solar wind speed exceeds 700km/sec, you may even have northward Bz and still have northern light. With high speeds there’s enough pressure on the magnetosphere so that negative Bz is less necessary to form the northern lights. With lower speeds southward Bz is always needed

I know there is a lot of discussion about the By and/or Bx being of importance for northern lights activity but I never heard of it nor read about it anywhere in an official paper. The only thing the By and Bx might tell you is the magnetic orientation of a magnetic structure in a CME which could sort of predict if there is a chance that the Bz will flip orientation if its been steady north or south for hours inside a CME core.

3 hours ago, Vancanneyt Sander said:

If solar wind speed exceeds 700km/sec, you may even have northward Bz and still have northern light. With high speeds there’s enough pressure on the magnetosphere so that negative Bz is less necessary to form the northern lights. With lower speeds southward Bz is always needed

This explains several instances I have seen. One in particular where our moderator Sam got some good pix during a positive Bz.  I don’t recall the wind speed precisely because I wasn’t paying attention at the time.  

There are actually several papers that do discuss this and I’m the process on organizing them so I can share them. Many of these papers are free and easy to find online.

It was first discussed in the 60’s and have since then been discussed here and there in papers. It has been found a correlation between absolute By higher than the Bz value. Usually this is buried in the papers as it isn’t resulting in as strong aurora than during a negative Bz.

As stated above, there are several scientists that have mentioned it in talks here and there. From what I have read, it’s not fully understood but it is a finding that has been observed several times. It’s also important to keep in mind that magnetic reconnection happens in several ways in our magnetosphere and not only in one way.

It's common to have strong aurora substorms even when the Bz is positive and the speed is less than 700 km/s. I posted an example in another thread during northward Bz. The speed during that occasion wasn't even over 600 km/s. I have checked webcams at the same time as this coupling occurs and it very often shows a substorm.

Here is one of the papers that discuss some of this: https://ntrs.nasa.gov/citations/20220016208

As I said, there are a lot more papers that touch on it and it is easy to miss since it often gets buried. It is often spoken about in seminars though. I can stop talking about it if people don't want to hear about it, just let me know in that case and I won't mention it.

Edited November 1, 2024Nov 1 by arjemma
Link to one paper

21 hours ago, Stine said:

Hello,

I don't know if my question is right here.

Also, please bear with me, if it is a stupid one.

I haven't been involved in speace weather for too long.

I keep reading over and over again that a negative BZ is necessary for the development of the aurora. I also understand the corresponding models and their explanations.

Now I have seen all of Tamitha Skov's courses and in her course "All about indices" she says that even a positive BZ with a negative BY can lead to aurora.

Michael Theusner also says this. In a WhatsApp group about spotting the aurora, a lady also reported seeing them with a positive BZ in northern germany. Some of the group said this was impossible.

I am confused.

Many models only assume a negative BZ and the BY is not discussed. 

Can someone explain to me what processes lead to auroras being visible even with positive BZ and negative BY? 

Or are there models that explain this clearly?

Please excuse me if it's a stupid question 🙈 and also my spotty English. It's not my native language.

 

I realise this has nothing much to do with your question but I wanted to give a positive response to a couple of points in your post 🙂

Firstly, no such thing as a stupid question in relation to solar weather and aurora. There's still many, many things even the best minds on Earth don't understand about the sun and its interactions with the wider universe so trying to understand some of the 'simpler' aspects is still challenging.

As for your English, I am a native English speaker and your typing is of a far greater standard than some of my closest friends and family. 

The community here is incredibly knowledgeable and supportive, no one will pick on you for asking a question that maybe they asked when they were new to the site too. People will simply answer your questions and refer you to good sources of information.

 

 

2 hours ago, arjemma said:

If people don't want to hear about it, just let me know in that case and I won't mention it.

Deffo interested to hear more! I know very little on the subject as a whole and have always believed that south Bz good, north Bz bad so if there are exceptions to this rule then it would be great to read more about them.

@arjemma side note, i always find your contributions interesting even when my smooth brain struggles to absorb all the info

2 hours ago, arjemma said:

I can stop talking about it if people don't want to hear about it, just let me know in that case and I won't mention it.

Please continue, the information you share is always hekpful! I always tell people each magnetic vector has an important roll that allows aurora to propagate in so many different ways. I think the last geomagnetic storm on the 28th had positive bz around 5-10 consistently and we still reached G2. A group I'm apart of couldn't figure out how that was possible with positive bz. I explained that the cause was a strong westward By but I struggled to describe the exact mechanisms in a way that the average civilian could easily understand. If I struggle to teach others then it probably means I haven't learned enough about the subject in the first place. I love helping people learn but I need to be able to teach others responsibly.

Amen @arjemma  I may not always follow your information at the time posted but it is always interesting and you provide references that others may verify independently should they desire.  Tnx!  I can usually grok 90% conceptually and that’s good enough to grasp concepts typically for me. 

20 hours ago, Vancanneyt Sander said:

If solar wind speed exceeds 700km/sec, you may even have northward Bz and still have northern light. With high speeds there’s enough pressure on the magnetosphere so that negative Bz is less necessary to form the northern lights. With lower speeds southward Bz is always needed

6 hours ago, arjemma said:

There are actually several papers that do discuss this and I’m the process on organizing them so I can share them. Many of these papers are free and easy to find online.

It was first discussed in the 60’s and have since then been discussed here and there in papers. It has been found a correlation between absolute By higher than the Bz value. Usually this is buried in the papers as it isn’t resulting in as strong aurora than during a negative Bz.

As stated above, there are several scientists that have mentioned it in talks here and there. From what I have read, it’s not fully understood but it is a finding that has been observed several times. It’s also important to keep in mind that magnetic reconnection happens in several ways in our magnetosphere and not only in one way.

It's common to have strong aurora substorms even when the Bz is positive and the speed is less than 700 km/s. I posted an example in another thread during northward Bz. The speed during that occasion wasn't even over 600 km/s. I have checked webcams at the same time as this coupling occurs and it very often shows a substorm.

Here is one of the papers that discuss some of this: https://ntrs.nasa.gov/citations/20220016208

As I said, there are a lot more papers that touch on it and it is easy to miss since it often gets buried. It is often spoken about in seminars though. I can stop talking about it if people don't want to hear about it, just let me know in that case and I won't mention it.

4 hours ago, Parabolic said:

Please continue, the information you share is always hekpful! I always tell people each magnetic vector has an important roll that allows aurora to propagate in so many different ways. I think the last geomagnetic storm on the 28th had positive bz around 5-10 consistently and we still reached G2. A group I'm apart of couldn't figure out how that was possible with positive bz. I explained that the cause was a strong westward By but I struggled to describe the exact mechanisms in a way that the average civilian could easily understand. If I struggle to teach others then it probably means I haven't learned enough about the subject in the first place. I love helping people learn but I need to be able to teach others responsibly.

I might weigh in with some recent discussion I saw on X with regards to the 2003 storm, as well as some regarding the 1972 one too:

The Newell coupling mentioned here is described in this paper, and is essentially a function that accounts for the total speed, and relates the total field strength to the clock angle in a specific way, with the function being given by:

Quote

v^4/3B_T^2/3sin^8/3(θ_c/2)

Where v is the speed and B_T is the total field strength (as usual), and θ_c refers to the clock angle. This isn't necessarily a perfect formula, but as per the paper has shown to have a very good correlation with the majority of the magnetospheric indices looked at, including Dst, Kp, and AE.

Edited November 1, 2024Nov 1 by Philalethes
paper

Just kidding with that reaction, maths was never my strong suit. Complex physics equations even more so 😁

It’s funny. The phi angle thing came up in a discussion between myself and my Aussie friend.  My eyes began to glaze over.  I realized that each of us has unique talents and really why on earth was I trying to assimilate knowledge that very likely won’t be of practical use to me?    It’s a bad habit of the geek in me.  Haha.   Hey @Philalethes since you’re still around have a quick look at the thing @Mantis started awhile ago pse.  I had a long-standing misunderstanding on particle behaviors that is likely well within your ability to understand and see if I am correct, or am missing something.   Tnx dude.  Later.   Oh yes. Ifya think 1400 km/sec is fast, the Aug 4 1972 blast was estimated at over 2800 km/sec!  No wonder our upper atmosphere took awhile to recover from that event!! 

Edited November 1, 2024Nov 1 by hamateur 1953
Tagged philalethes. Aug 4 1972

Oh wow...I didn't expect so many answers! 
Thank you to each one of you for your thoughts and time!
Tomorrow I will have more time to read through the individual answers calmly and to understand them. Thanks!

9 hours ago, BlueRoom said:

Deffo interested to hear more! I know very little on the subject as a whole and have always believed that south Bz good, north Bz bad so if there are exceptions to this rule then it would be great to read more about them.

@arjemma side note, i always find your contributions interesting even when my smooth brain struggles to absorb all the info

9 hours ago, Parabolic said:

Please continue, the information you share is always hekpful! I always tell people each magnetic vector has an important roll that allows aurora to propagate in so many different ways. I think the last geomagnetic storm on the 28th had positive bz around 5-10 consistently and we still reached G2. A group I'm apart of couldn't figure out how that was possible with positive bz. I explained that the cause was a strong westward By but I struggled to describe the exact mechanisms in a way that the average civilian could easily understand. If I struggle to teach others then it probably means I haven't learned enough about the subject in the first place. I love helping people learn but I need to be able to teach others responsibly.

Thank you, both of you!

It's very easy to oversimplify space weather and how it works cause it's easier. The thing is that space weather and the affects on earth is far more complex than only a positive or negative Bz. If it was that simple, all the mysteries involving space weather would have been solved already. The more complex something is, the easier it is for our brains to find shortcuts. A shortcut in this case is thinking that the Bz is all that matters. You were correct in thinking about the strong westward By during the last storm. The absolute value of By were a lot stronger than the Bz several times during that evening which resulted in a lot of aurora even though the Bz was northward. It was a very good example and was easy to validate with webcam images together with the data.

It's the same for me, I want to be able to have all my sources ready with quotes before I describe this further so there isn't any misunderstandings.

I just hope that what I share isn't thought of as "psuedoscience" and has me banned in the end so I will try to be very careful and link sources when I bring it up.

4 hours ago, Philalethes said:

The Newell coupling mentioned here is described in this paper, and is essentially a function that accounts for the total speed, and relates the total field strength to the clock angle in a specific way, with the function being given by:

Quote

v^4/3B_T^2/3sin^8/3(θ_c/2)

Where v is the speed and B_T is the total field strength (as usual), and θ_c refers to the clock angle. This isn't necessarily a perfect formula, but as per the paper has shown to have a very good correlation with the majority of the magnetospheric indices looked at, including Dst, Kp, and AE.

That paper is also very good, thanks for the link!

All this just shows how complex all this is. Sure, the Bz component is important in most cases but it is not acting alone.

2 hours ago, Stine said:

Oh wow...I didn't expect so many answers! 
Thank you to each one of you for your thoughts and time!
Tomorrow I will have more time to read through the individual answers calmly and to understand them. Thanks!

No worries, I like that you opened a topic about this cause it's a bit sad that it is overlooked so much. As I said above, I will try to sort through the papers I have saved, and then try to make some sense of all this in a way that is more easy to understand. Space weather is a really complex field.

30 minuten geleden, arjemma zei:

I just hope that what I share isn't thought of as "psuedoscience" and has me banned in the end so I will try to be very careful and link sources when I bring it up.

No of course you aren't getting a ban you silly! This is great information. Do not have time to go deeper into it at the moment but for sure it a very interesting subject and definitely not far fetched. I am sure we all know the difference between pseudoscience and well yeah... normal science? Haha.

3 hours ago, hamateur 1953 said:

The phi angle thing came up in a discussion between myself and my Aussie friend.  My eyes began to glaze over.

Heh, coincidentally there's a good page on the clock angle by the ASWFC here (it also has a similar approximating function to the one above listed near the bottom), so clearly they know a thing or two about space weather in Straya. Note that the clock angle isn't the same as the phi angle, which is rather in the xy-plane; and I don't think it's the same as the theta angle listed on e.g. ACE and here on SWL either, but I'm not entirely sure what that actually measures (I think it's the angle between Bx and Bz, the "latitude" of the Solar wind).

At least the clock angle is what's described on that site, i.e. the angle formed by the By and Bz; when plugging it into the sin^(8/3)(θ_c/2) expression above you get 1 for an angle of 180° (all negative Bz, no By), but still ~0.4 for angles of 90° and 270° (i.e. all By and no Bz), hence why even strong and fast CMEs can cause activity despite a northward Bz. Only for a clock angle of 0° (purely positive Bz with zero By) do you get 0.

3 hours ago, hamateur 1953 said:

Ifya think 1400 km/sec is fast, the Aug 4 1972 blast was estimated at over 2800 km/sec!  No wonder our upper atmosphere took awhile to recover from that event!! 

Important to note that the 1400 km/s there refers to the sheath/CME speed registered at L1, while the 2800 km/s one from 1972 I believe refers to the average speed at 1 au, so they can't really be compared directly like that; the 1972 one would likely have started out much faster and ended slower. I don't actually know what speed the 1972 one arrived here with, since as I've lamented before the data is lacking from OMNIWeb, but it was probably still significantly faster than the 2003 one, which only had an average speed of ~2200 km/s. Suffice to say that with such a blast we'd be virtually guaranteed a lot of geomagnetic activity no matter what.

19 minutes ago, Marcel de Bont said:

No of course you aren't getting a ban you silly! This is great information. Do not have time to go deeper into it at the moment but for sure it a very interesting subject and definitely not far fetched. I am sure we all know the difference between pseudoscience and well yeah... normal science? Haha.

Haha true, my brain is not braining 😂

1 hour ago, Philalethes said:

Heh, coincidentally there's a good page on the clock angle by the ASWFC here (it also has a similar approximating function to the one above listed near the bottom), so clearly they know a thing or two about space weather in Straya. Note that the clock angle isn't the same as the phi angle, which is rather in the xy-plane; and I don't think it's the same as the theta angle listed on e.g. ACE and here on SWL either, but I'm not entirely sure what that actually measures (I think it's the angle between Bx and Bz, the "latitude" of the Solar wind).

At least the clock angle is what's described on that site, i.e. the angle formed by the By and Bz; when plugging it into the sin^(8/3)(θ_c/2) expression above you get 1 for an angle of 180° (all negative Bz, no By), but still ~0.4 for angles of 90° and 270° (i.e. all By and no Bz), hence why even strong and fast CMEs can cause activity despite a northward Bz. Only for a clock angle of 0° (purely positive Bz with zero By) do you get 0.

Important to note that the 1400 km/s there refers to the sheath/CME speed registered at L1, while the 2800 km/s one from 1972 I believe refers to the average speed at 1 au, so they can't really be compared directly like that; the 1972 one would likely have started out much faster and ended slower. I don't actually know what speed the 1972 one arrived here with, since as I've lamented before the data is lacking from OMNIWeb, but it was probably still significantly faster than the 2003 one, which only had an average speed of ~2200 km/s. Suffice to say that with such a blast we'd be virtually guaranteed a lot of geomagnetic activity no matter what.

Geomagnetic activity in that particular case includes the detonation of over 10,000 magnetic sea mines   I should repost that four hour class by Tamitha Skov.  Was one of her best imho 

Its in the “other” section on the main board entitled “ Interesting Four Hour Class”  I think I entitled it.  For those who may be intrigued by that comment.  She explains how the “ magnetic crochet”. Her term I believe, was actually responsible in a complicated manner for that very bizarre occurrence.  

I read through all the posts and also the links.

I still lack more basic physical knowledge to understand it 🙈

I remember learning about vectors in math and physics at school. Can I imagine the interaction between BZ and BY in a similar way?

"If several forces act on a body, they are added vectorially (taking their direction into account) to form a resulting force."

I'm working on it, will deal with Substorms, and then hope to be able to better understand what was written in the texts.

But what I'm already taking with me is that it's not impossible to see the northern lights at a high enough speed even with a positive BZ and the statement that the BZ has to be negative isn't generally true.

16 hours ago, Stine said:

I read through all the posts and also the links.

I still lack more basic physical knowledge to understand it 🙈

I remember learning about vectors in math and physics at school. Can I imagine the interaction between BZ and BY in a similar way?

"If several forces act on a body, they are added vectorially (taking their direction into account) to form a resulting force."

I'm working on it, will deal with Substorms, and then hope to be able to better understand what was written in the texts.

But what I'm already taking with me is that it's not impossible to see the northern lights at a high enough speed even with a positive BZ and the statement that the BZ has to be negative isn't generally true.

The way I look at it is, when even the scientists who study solar weather don't fully understand everything about it us 'normies' can only do our best to understand a fraction of the information they have. 

That doesn't mean we're stupid of course, just that we can only do our best with the skills we have. 

However as a group we can expand our knowledge in ways we never thought possible. 

 

 

On 11/3/2024 at 12:52 PM, Stine said:

I'm working on it, will deal with Substorms, and then hope to be able to better understand what was written in the texts.

Earth's magnetic field is continuously buffeted by clouds of energetic, electrically charged, particles – mainly electrons and protons - which are carried in the solar wind or released during huge explosions on the Sun, known as coronal mass ejections.

The magnetic bubble that surrounds Earth – the magnetosphere – generally protects the planet from these particles, but the shield is sometimes breached, enabling particles and energy to accumulate in the magnetotail.

This stored energy is subsequently released as the result of a sudden rearrangement of the magnetic field lines. Electrified particles then race along the field lines and enter the atmosphere above the magnetic poles, creating bright rings of light in the form of colourful auroras.

Although this basic process has been recognised for many years, there has been no scientific consensus about where the geomagnetic substorms are initiated.

One question concerned whether they are caused by a sudden disruption of electric current about 64 000 km from the planet, or by a process called magnetic reconnection which occurs much further down the magnetotail, at a distance of around 125 000 – 200 000 km.

Although recent research appears to favour the magnetic reconnection mechanism, a major problem with this theory involves the rapid onset of auroras after the sudden realignment of the magnetic field lines.

According to established theory, the energy from the reconnection event is carried by Alfvén waves - a type of magnetic wave that propels the charged particles in the plasma towards and away from Earth.

However, these Alfvén waves travel quite slowly, reaching Earth after a travel time of about 250 seconds. They cannot account for some observations of substorm events which indicate that auroras intensify less than one minute after the onset of reconnection – much earlier than expected. This discrepancy led to the suggestion that another, faster, type of wave – known as a kinetic Alfvén wave (KAW) – might also be generated during a substorm.
Unlike ordinary Alfvén waves, which move both ions and electrons towards Earth at 500 - 1000 km/s, kinetic Alfvén waves influence only electrons. This enables them to travel much faster through the plasma, at speeds of several thousand kilometres per second.

In an effort to investigate these questions, Michael Shay, a professor at the Bartol Research Institute in the Department of Physics and Astronomy at the University of Delaware, began a series of simulations on a Cray XE6, one of the most powerful computers in the world. Located at the National Energy Research Scientific Computing Center, the HOPPER supercomputer crunched away at solving fluid equations that simulated the behaviour of individual particles energised by reconnection.

The simulations confirmed that KAWs could be generated by reconnection and then propagate rapidly away from the site of the explosion, reaching Earth in less than one minute. They also confirmed that KAWs carry enough energy to intensify auroras.

Further verification of this result came from Jonathan Eastwood, a Research Fellow at The Blackett Laboratory, Imperial College London, who began to trawl through data sent back by the four Cluster spacecraft during their decade-long exploration of near-Earth space.

Eastwood was sifting through the data returned by the Fluxgate Magnetometer (FGM) and the Electric Fields and Waves (EFW) instrument on each spacecraft, in a search for measurements which coincided with magnetic reconnection events in the magnetotail.

source: https://sci.esa.int/web/cluster/-/49107-ultrafast-substorm-auroras-explained

 

Per @Philalethes suggestion, I am taking this topic to this thread. It seems the best fitting. My apology if I misunderstood the feedback provided by all involved in the discussion. Just a quick recap of what is being discussed:

On 2/26 between 8 and 10 pm EST (1:00 and 3:00 UT), there was a very bright and quite dynamic aurora seen on an aurora web cam from Millinocket Lake in Maine, 45.74N. The space weather parameters were: Bt stable at approximately 12nT, HP in 20s, Kp 2.33, wind speed between 540 and 580 km/s, density fluctuating between 10 and 20, Bz fluctuating but mostly close to zero or positive with values sometimes briefly over +7nT while the substorm was going on. @Samrau hypothesized that this might be due to strong By and referred me to this thread for more discussion on aurora displays with positive Bz. As pointed out by @Philalethes, the Newell coupling does provide an explanation of this strong aurora display based on the time derivative of the magnetic flux v^4/3B_T^2/3sin^8/3(θ_c/2), where v is the speed and \theta_c is the clock angle defined by Bz and By, zero when pointing up (N). I went into the archives for the IMF for this above time period and, indeed, found out that during the substorm, By was positive and mostly much larger than |Bz|. This means that the clock angle was close to 90 degrees, making that sin term (sqrt(2)/2)^8/3 ~ 0.4, meaning we still have a large rate of the magnetic flux, hence the aurora.

This begs one question. Shall we add this quantity to the SWL board? It is computable from Bt, Bz, By, and speed. Should be easy to add. Having access to this could mean the difference between catching and not catching a display that is challenging to identify from established indices. Of course, the challenge would be how to color the values - e. g., the thresholds at which it would predict a storm.

And another question: Do indices, such as AE or HP take into account this quantity? Are there indices that do take this into account?

P.S.: Just a technical remark, that sin can be negative. Raising a negative number to a fraction is technically not defined in the real domain. Thus, I assume that the sin should be in absolute value (?) It should not matter whether By > 0 or By < 0, right?

@Jesterface23 @Peogauuia @NightSky @hamateur 1953 @arjemma

Edited March 1Mar 1 by JessicaF

19 minutes ago, JessicaF said:

On 2/26 between 8 and 10 pm EST (1:00 and 3:00 UT), there was a very bright and quite dynamic aurora seen on an aurora web cam from Millinocket Lake in Maine, 45.74N. The space weather parameters were: Bt stable at approximately 12nT, HP in 20s, Kp 2.33, wind speed between 540 and 580 km/s, density fluctuating between 10 and 20, Bz fluctuating but mostly close to zero or positive with values sometimes briefly over +7nT while the substorm was going on.

I will assume you thus mean between 01:00 and 03:00 UTC on 02-27 in the data, as that seems to fit most of the parameters you describe (although looking at the RTSW it seems like the density is hovering around 2-4 rather than 10-20 during that time, so not entirely sure, feel free to clarify, if it's a different time it would of course change everything); to be clear, this is the data in question that I'm looking at, a 6-hour interval centered on the 2-hour interval you mention:

And for the substorm(s) I thus assume it was some of the smaller leftmost drops here, presumably that around 02:00:

Not a huge storm by the looks of it, but at a geomagnetic latitude of ~55° the location in question I'm guessing it would still be sufficient to produce some visible aurora there, though I probably wouldn't have guessed there would have been anything "very bright" there if you had just given me the data and nothing else. Do you perhaps have the footage to share? I'm not that well versed when it comes to photography, but I know there are ways to photograph/film aurora so it looks a lot brighter than it looks to human eyes directly, so maybe there was some of that going on too.

59 minutes ago, JessicaF said:

@Samrau hypothesized that this might be due to strong By and referred me to this thread for more discussion on aurora displays with positive Bz. As pointed out by @Philalethes, the Newell coupling does provide an explanation of this strong aurora display based on the time derivative of the magnetic flux v^4/3B_T^2/3sin^8/3(θ_c/2), where v is the speed and \theta_c is the clock angle defined by Bz and By, zero when pointing up (N). I went into the archives for the IMF for this above time period and, indeed, found out that during the substorm, By was positive and mostly much larger than |Bz|. This means that the clock angle was close to 90 degrees, making that sin term (sqrt(2)/2)^8/3 ~ 0.4, meaning we still have a large rate of the magnetic flux, hence the aurora.

In this particular case it could certainly have played some role in sustaining some geomagnetic activity, although from the AL and AE it seems like the bigger storms that happened later that day didn't occur until after a foray into strongly negative Bz. But your analysis is sound, in principle a decently strong By can in fact make up for a lack of Bz, and the coupling is indeed ~0.4 of what it would be for a completely southwards clock angle. The higher speed also helps. That being said, typically you really need a stronger field, and even a faster speed too, before you start seeing solid activity that way. If you calculate the coupling for the given parameters, being generous and setting the By to 10 and the speed to 650 (and Bz to 0), the resulting coupling is 10,400, whereas more normal parameters like e.g. a By of 2, Bz of -2, and speed of 425, would yield a coupling of ~5200, so we can see that there's some enhancement, but not overly much. You'd e.g. match it with a By of 5, Bz of -5, and speed of 450 (which isn't all that bad when you think about it). For comparison the May storm of 2024 peaked at a coupling of ~73,700.

With that in mind, I'd also note that there had been some geomagnetic activity preceding this, with some prolonged periods of -Bz earlier on 02-26; it does look like it had calmed down before activity started after midnight, but it's not impossible that it could have primed the magnetosphere:

1 hour ago, JessicaF said:

This begs one question. Shall we add this quantity to the SWL board? It is computable from Bt, Bz, By, and speed. Should be easy to add. Having access to this could mean the difference between catching and not catching a display that is challenging to identify from established indices. Of course, the challenge would be how to color the values - e. g., the thresholds at which it would predict a storm.

I personally wouldn't mind seeing it either, but that's something for the admins to consider I guess. Newell's paper is certainly physically sound, and the coupling shows good correlations with various magnetospheric indices, as mentioned previously in the thread. In the paper he even quips:

Excluding the often used but noncompetitive Bz, probably the candidate coupling functions most often encountered in current space physics research are (1) the half-wave rectifier; (2) the ɛ parameter of Perreault and Akasofu [1978] (ɛ = vB²sin²(θ_c/2)); and (3) the Kan-Lee electric field (EKL = vB_T²sin²(θ_c/2)) [Kan and Lee, 1979]. Here the IMF clock angle is defined by θ_c = arctan(By/Bz). All three consistently produce better results than Bz does alone.

So at least considering coupling functions like these, especially the Newell coupling in question, is probably a good idea. One can also get some ideas about the relations from the plots and tables in the paper, like e.g. the ones comparing the coupling to AE or Kp. A rough estimate based on what it looks like to me (don't hold me to it) would be that you start seeing some minor activity with potential substorming when the coupling approaches 10,000 or so, and minor geomagnetic storm conditions around 15,000.

Also, an important note about the coupling function itself that I got confused myself previously due to a misleading statement in the abstract: the "T" in B_T likely stands for "transverse" or "tangential", because in the literature it's very explicitly used to refer to the field strength in the yz-plane exclusively, and even contrasted explicitly with just B (which is what we call Bt, but is not B_T; confusing!), the total field strength overall. In other words, you calculate the function entirely from the By and the Bz (and the speed, of course), where instead of B(t) = sqrt(Bx² + By² + Bz²) you just do B_T = sqrt(By² + Bz²). The Bx is ignored entirely.

1 hour ago, JessicaF said:

And another question: Do indices, such as AE or HP take into account this quantity? Are there indices that do take this into account?

Nope, the indices we generally look at are calculated from geomagnetic measurements (including but not limited to Kp/Ap, AE, and Dst), quite different from calculating a quantity from solar wind parameters; the idea of a coupling function is more to both be some physical representation of the reconnection process and to correlate as well as possible with those geomagnetic indices, which the paper does a lot of analysis on, showing that the Newell coupling correlates more strongly than any of the other coupling functions investigated for 9 of the 10 indices looked at (the remaining one interestingly being the Dst; section [41] is on that, with a very intriguing result).

2 hours ago, JessicaF said:

P.S.: Just a technical remark, that sin can be negative. Raising a negative number to a fraction is technically not defined in the real domain. Thus, I assume that the sin should be in absolute value (?) It should not matter whether By > 0 or By < 0, right?

As you take the sine of half the clock angle, the domain becomes [0°, 180°], where the sine ranges from 0 up to 1 and back down to 0 again without ever becoming negative regardless of what By and Bz are or whether they're positive or negative, avoiding any such problems! For example, in the cases where Bz is 0, the clock angle becomes 90° or 270° depending on the sign of By; in the two cases you end up with sin(45°) or sin(135°), which both yield the exact same value, 1/sqrt(2), due to symmetry. This is indeed a perfectly valid positive number that we can raise to any fractional exponent, and raising it to 8/3 we end up with ~0.4; all is well.

OP removed silly remarks. 😊

Edited March 1Mar 1 by hamateur 1953
Wasn’t awake.

I wanted to add the Oct. geomagnetic had quite a lot of negative By along side the strong southward Bz. I also included Dst and the 3hr Kp from that day.

Forgot to add the legend but Blue is By, Orange is Bz, and Green is Bt. Just realized the title for this topic and what I posted doesn't exactly pertain to the subject. Oct. 28th might be a good example of postive Bz and negative By. I'll put together something when the sun rises for me.

Edited March 1Mar 1 by Parabolic
included important info and being a sleepy head not realizing what the topic is