<p>As I have answered <a href="https://astronomy.stackexchange.com/questions/58391/aurora-borealis-northern-lights/58398#58398">here</a>:</p>
<blockquote>
<p>What the correlation does show is that there are more observations around the equinoxes, but we need to wonder why such is the case.</p>
</blockquote>
<p>and later, with a slight adjustment in text:</p>
<blockquote>
<p>One doesn’t have more (or less) chances to see auroras [around] the equinox. The Earth–Sun connection is always as strong (or weak) as usual. But in the Northern Hemisphere, because the night is ever so slightly longer after the September equinox, the odds are slightly better, and inversely, they are better the week before the March equinox, for the very same reason. So, deep down, it’s not whether there will be northern lights or not, but whether the night will be long enough to get an observer the chance to see them…</p>
<p>…which means the best period to see northern lights would be around the December solstice because that’s the longest night of the year. But it’s also close to being the coldest night of the year (which in in January or February), so less people will be outside just waiting for northern lights to be seen.</p>
</blockquote>

CC BY-SA 4.0
2024-08-19T23:58:01.167Z
But a statistic about the magnetic index is not about visibility. Thus the data linked in the question is not only visible lights, but magnetic activity, e. g. see the graph i copied. While something similar like you argue was my first thought, too, I believe the data don't support it
planetmaker
CC BY-SA 4.0
2024-08-20T07:28:41.840Z
Why do you discount the orientation of the Earth's magnetic field with respect to to the Sun? This would seem to be important and a probabl explanation of a modulation over the year.
ProfRob
CC BY-SA 4.0
2024-08-22T02:05:24.100Z
@ProfRob: I don’t *discount* it; I just didn’t think of it…
Pierre Paquette
CC BY-SA 4.0
2024-08-19T23:43:52.273Z
2024-08-19T23:43:52.273Z
Pierre Paquette
<p>I had to look this one up. It is a genuine effect, not just an observational bias. The frequency of aurorae has been recorded with automatic cameras on timescales of a decade, showing that the occurrence rate of aurorae in Tromso is greater in October and February than in the months in between (e.g. <a href="https://www.researchgate.net/figure/Monthly-occurrence-rate-of-aurora-for-the-past-10-years-Each-panel-shows-from-top-to_fig4_360994002" rel="nofollow noreferrer">Nanjo et al. 2022</a>). The data outside this range are limited by hours of daylight.</p>
<p>Others
have used geomagnetic data on the interplanetary magnetic field data and geomagnetic indices to confirm a modulation that peaks around the autumn and spring equinoxes (<a href="https://ui.adsabs.harvard.edu/abs/2012JGRA..11711222Z/abstract" rel="nofollow noreferrer">Zhao and Zong 2012</a>; <a href="https://www.swsc-journal.org/articles/swsc/full_html/2020/01/swsc190051/swsc190051.html" rel="nofollow noreferrer">Lockwood et al. 2020a</a>). The latter paper gives lots of references to previous work on the phenomenon.</p>
<p>The proposed explanation is the <a href="https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JA078i001p00092" rel="nofollow noreferrer">Russell-McPherron</a> (R-M) effect. I can only read the abstract, but a summary would seem to be that because of the Earth's tilt and the vague alignment of the magnetic and rotation axes, there is an annual modulation of the orientation of the geomagnetic field with respect to to the Sun. This changing alignment then makes it more or less likely that a given level of solar activity will result in aurorae.</p>
<p>It's a bit complicated though (<a href="https://www.swsc-journal.org/articles/swsc/full_html/2020/01/swsc190053/swsc190053.html" rel="nofollow noreferrer">Lockwood et al. 2020b</a>)</p>
<blockquote>
<p>The effect arises because geomagnetic activity is driven by southward field in the Geocentric Solar Magnetospheric (GSM) frame (<span class="math-container">$[B_Z]_{GSM} &lt; 0$</span>) and the rotation of the GSM frame relative to the Geocentric Solar Equatorial (GSEQ) frame around their common X axis (i.e. in their common YZ plane) depends of the fraction of the year, F, and the Universal Time, UT. Paper 1 demonstrated the R-M effect in a number of ways, but the strongest evidence is that the “favoured” equinox, at which geomagnetic activity is enhanced, was shown to depend on the polarity of the dawn-dusk component of the Interplanetary Magnetic Field in GSEQ <span class="math-container">$[B_Y]_{GSEQ}$</span>, with the enhancement being only at the September equinox for <span class="math-container">$[B_Y]_{GSEQ} &gt; 0$</span> and only at the March equinox for <span class="math-container">$[B_Y]_{GSEQ} &lt; 0$</span>.</p>
</blockquote>

CC BY-SA 4.0
2024-08-20T07:54:39.550Z
2024-08-23T15:25:55.913Z
2024-08-23T15:25:55.913Z
ProfRob
<p>Following the link of <a href="https://astronomy.stackexchange.com/questions/58391/aurora-borealis-northern-lights">this question</a> it shows at least seemingly sound statistics that the magnetic activity is <a href="https://seetheaurora.com/aurora-stats-and-averages" rel="noreferrer">seasonally dependent</a>.
<a href="https://i.sstatic.net/FB9yCsVo.png" rel="noreferrer"><img src="https://i.sstatic.net/FB9yCsVo.png" alt="Monthly magnetic index" /></a></p>
<p>So far it was my believe that the magnetic activity depends on the Sun and the interaction of the Solar Wind with Earth's magnetosphere - thus processes which do not mind the season much. What are the reasons behind this behaviour? The only reason I can think of is the different coupling with the interplanetary field depending on the alignment of the Earth's axis which varies by season. This seems to be somewhat confirmed, e.g. by <a href="https://angeo.copernicus.org/articles/39/929/2021/" rel="noreferrer">Marques de Souza Franco et al (2021)</a>, though their evidence on the coupling depends a bit on what features they look at.</p>
<p>Does there exist a comprehensive summary of current understanding the effects involved and their causes and underlaying physics, possibly in a nice review paper?</p>

<earth><magnetic-field><aurora><solar-storm>
Seasonal variation of aurora and magnetic activity
planetmaker
# Seasonal variation of aurora and magnetic activity

(Asked by: username_0 on 2024-08-19)

Following the link of [this question](https://astronomy.stackexchange.com/questions/58391/aurora-borealis-northern-lights) it shows at least seemingly sound statistics that the magnetic activity is [seasonally dependent](https://seetheaurora.com/aurora-stats-and-averages).
[![Monthly magnetic index](https://i.sstatic.net/FB9yCsVo.png)](https://i.sstatic.net/FB9yCsVo.png)

So far it was my believe that the magnetic activity depends on the Sun and the interaction of the Solar Wind with Earth's magnetosphere - thus processes which do not mind the season much. What are the reasons behind this behaviour? The only reason I can think of is the different coupling with the interplanetary field depending on the alignment of the Earth's axis which varies by season. This seems to be somewhat confirmed, e.g. by [Marques de Souza Franco et al (2021)](https://angeo.copernicus.org/articles/39/929/2021/), though their evidence on the coupling depends a bit on what features they look at.

Does there exist a comprehensive summary of current understanding the effects involved and their causes and underlaying physics, possibly in a nice review paper?

**Tags:** earth, magnetic-field, aurora, solar-storm

---

## 2 answers

### Answer by username_2 on 2024-08-20. Score: 6 | Accepted answer

I had to look this one up. It is a genuine effect, not just an observational bias. The frequency of aurorae has been recorded with automatic cameras on timescales of a decade, showing that the occurrence rate of aurorae in Tromso is greater in October and February than in the months in between (e.g. [Nanjo et al. 2022](https://www.researchgate.net/figure/Monthly-occurrence-rate-of-aurora-for-the-past-10-years-Each-panel-shows-from-top-to_fig4_360994002)). The data outside this range are limited by hours of daylight.

Others
have used geomagnetic data on the interplanetary magnetic field data and geomagnetic indices to confirm a modulation that peaks around the autumn and spring equinoxes ([Zhao and Zong 2012](https://ui.adsabs.harvard.edu/abs/2012JGRA..11711222Z/abstract); [Lockwood et al. 2020a](https://www.swsc-journal.org/articles/swsc/full_html/2020/01/swsc190051/swsc190051.html)). The latter paper gives lots of references to previous work on the phenomenon.

The proposed explanation is the [Russell-McPherron](https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/JA078i001p00092) (R-M) effect. I can only read the abstract, but a summary would seem to be that because of the Earth's tilt and the vague alignment of the magnetic and rotation axes, there is an annual modulation of the orientation of the geomagnetic field with respect to to the Sun. This changing alignment then makes it more or less likely that a given level of solar activity will result in aurorae.

It's a bit complicated though ([Lockwood et al. 2020b](https://www.swsc-journal.org/articles/swsc/full_html/2020/01/swsc190053/swsc190053.html))

> The effect arises because geomagnetic activity is driven by southward field in the Geocentric Solar Magnetospheric (GSM) frame ($[B\_Z]\_{GSM} < 0$) and the rotation of the GSM frame relative to the Geocentric Solar Equatorial (GSEQ) frame around their common X axis (i.e. in their common YZ plane) depends of the fraction of the year, F, and the Universal Time, UT. Paper 1 demonstrated the R-M effect in a number of ways, but the strongest evidence is that the “favoured” equinox, at which geomagnetic activity is enhanced, was shown to depend on the polarity of the dawn-dusk component of the Interplanetary Magnetic Field in GSEQ $[B\_Y]\_{GSEQ}$, with the enhancement being only at the September equinox for $[B\_Y]\_{GSEQ} > 0$ and only at the March equinox for $[B\_Y]\_{GSEQ} < 0$.


---

### Answer by username_1 on 2024-08-19. Score: 0

As I have answered [here](https://astronomy.stackexchange.com/questions/58391/aurora-borealis-northern-lights/58398#58398):

> What the correlation does show is that there are more observations around the equinoxes, but we need to wonder why such is the case.

and later, with a slight adjustment in text:

> One doesn’t have more (or less) chances to see auroras [around] the equinox. The Earth–Sun connection is always as strong (or weak) as usual. But in the Northern Hemisphere, because the night is ever so slightly longer after the September equinox, the odds are slightly better, and inversely, they are better the week before the March equinox, for the very same reason. So, deep down, it’s not whether there will be northern lights or not, but whether the night will be long enough to get an observer the chance to see them…
>
> …which means the best period to see northern lights would be around the December solstice because that’s the longest night of the year. But it’s also close to being the coldest night of the year (which in in January or February), so less people will be outside just waiting for northern lights to be seen.

> **Comments:**
> **username_0**: But a statistic about the magnetic index is not about visibility. Thus the data linked in the question is not only visible lights, but magnetic activity, e. g. see the graph i copied. While something similar like you argue was my first thought, too, I believe the data don't support it  
> **username_2**: Why do you discount the orientation of the Earth's magnetic field with respect to to the Sun? This would seem to be important and a probabl explanation of a modulation over the year.  
> **username_1**: @ProfRob: I don’t *discount* it; I just didn’t think of it…  

---

