IMF - BT strength

[Jay-B]

What is the highest ever recorded IMF strength on record? I see it says >30 is very strong but how high can it actually go?

[Vancanneyt Sander]

Well we keep track on solar wind stats in a database (not implemented publicly on SWL) with data since 1996. 

Strongest Bt: 81,3nT - Bz -78,17nT - recorded November 6th 2001. Check the archive:

[Philalethes]

2 hours ago, Jay-B said:

What is the highest ever recorded IMF strength on record? I see it says >30 is very strong but how high can it actually go?

I also checked OMNIWeb, which has measurements back to 1963 (although I don't know how reliable they are compared to contemporary data), and it also yields the same date Sander mentions above as the one with the strongest Bt, albeit slightly lower values due to being hourly averages in the OMNIWeb data. There the maximum hourly average is listed as a Bt of 62.0 with a Bz of -57.0. The second strongest is June 6 of 1979 with a Bt of 54.8, and a Bz of just -3.0.

Some of the stronger storms aren't recorded properly in the older data though, probably due to knocking out the measurement equipment; the superstorm of 1972, for example, is entirely blanked out. There has however been estimates of it based on other measurements, but you can't really call that a proper record. In this paper they write:

Quote

 

[...] the expected magnetic field strength at 1 AU would be 103 nT and a maximum interplanetary electric field of 229 mV/m.

[...]

Unfortunately, there was no measurement of the magnetic fields for the ejecta for the 1972 event at 1 AU. There was, however, a deep space magnetic field measurement made by Pioneer 10 at 2.2 AU. The field strength of the magnetic field was ~15 nT. Assuming an r^(-2) drop-off of field intensity with radial distance and no superradial expansion (due to high internal pressures), the field would have been ~73 nT at 1 AU. With greater internal pressures, the field strength could have been higher.

 

So even though we can't know for sure, it was definitely up there and contending with the 2001 one. I've also seen some estimates of the Carrington Event at around 100 Bt, so it's probably in this 70-100 Bt range the most extreme events will tend to fall in. Perhaps even more extreme events could go higher too, but then we're probably talking events that only happen every few thousands of years or more.

[Jesterface23]

I guess it depends on how close you are to the Sun.

For near 0.96 AU, STEREO A would win with the July 23rd, 2012 event at B total strength of 109.26nT.

 

My data may not be up to date, but the Parker Solar Probe reached B total strength of 638.82nT at about 0.055AU in 2021.

That November 6th, 2001 event does look to be the strongest for L1 over the past ~30 years with L1 for the event being at around 0.98AU.

[Jay-B]

Wow! That is more than I expected! This gives me hope for an Ohio light show some day!! 

[Jay-B]

15 hours ago, Philalethes said:

The second strongest is June 6 of 1979 with a Bt of 54.8, and a Bz of just -3.0.

How disappointing would that have to be? Lol  just -3 bz. 

[Vancanneyt Sander]

17 uren geleden, Philalethes zei:

The second strongest is June 6 of 1979 with a Bt of 54.8, and a Bz of just -3.0.

Well I do have to correct 😉 second strongest is also in 2001 (March 31st) with Bt maxed at 72nT (Bz min -47,41nT). Third place, also 2001 (November 24th) Bt maxed at 70,81 (Bz min -49,29). 
your date is at 11th place in the top 10 since 1996.

17 uren geleden, Philalethes zei:

Some of the stronger storms aren't recorded properly in the older data though, probably due to knocking out the measurement equipment

True, ACE was knocked out several times during the strongest storms (especially during proton storms). I still remember those days. We had the MTOF monitor (instrument on SOHO) as a bit of a backup to have an indication of solar wind speed but it was with great delay.

[Philalethes]

3 hours ago, Jay-B said:

How disappointing would that have to be? Lol  just -3 bz. 

Not ideal for aurora hunting, heh. Here are the surrounding values:

Quote

1979 157 15  11.5  -3.7
1979 157 16  11.6  -3.1
1979 157 17  11.0   2.6
1979 157 18  12.6   6.4
1979 157 19  11.3  -1.7
1979 157 20  38.5   9.3
1979 157 21  54.8  -3.0
1979 157 22  47.3  23.7
1979 157 23  38.5  14.4
1979 158  0  32.0   6.6
1979 158  1  23.9   6.6
1979 158  2  23.4  -1.2
1979 158  3  24.4  -7.0
1979 158  4  16.8  -7.4
1979 158  5  15.1  -2.5

Days 157 and 158 are June 6 and 7 respectively. That entire storm seemed to have been mostly northward, just like the one in 1972; if you get something like that mostly southward instead you get the strong events like e.g. 2003. Another infamous storm was the 1989 one, which this paper mentions to have been estimated at a Bs (which is the same as Bz, except for positive values being evaluated to zero) of around -50 nT at its peak, using a scaling factor found empirically from the 2003 storm.

Perhaps more interesting is to look at the various geomagnetic indices, like the Kp-index (and its related indices like ap and Ap), the Dst-index (or its more contemporary and high-resolution version, SYM-H), and/or the auroral electrojet index, to get more insight into how Earth was actually impacted by the events. For the Kp-index alone it's not easy to discern between the strongest events since it maxes out at a certain point, but if you use the daily average of the equivalent ap-index, which is the Ap-index, you can get a view of which storms had fairly persistent geomagnetic activity over many hours (although it might be better to do a running average of the ap-index). If we e.g. check out the instances where the Ap-index was over 200, we get just 11 different days:

Quote

['1941', '03', '01', '207']
['1941', '07', '05', '222']
['1941', '09', '18', '232']
['1946', '03', '28', '213']
['1959', '07', '15', '236']
['1960', '04', '01', '241']
['1960', '10', '06', '203']
['1960', '11', '13', '280']
['1986', '02', '08', '202']
['1989', '03', '13', '246']
['2003', '10', '29', '204']

Not unexpectedly we find both the 1989 and 2003 event here, and several days around the height of the humongous SC19 in 1959 and 1960. Curiously we don't find the 2001 there, since despite the large peak in the Bt and Bz the geomagnetic activity still didn't reach that high (but according to Wikipedia it still led to "vivid aurorae as far south as Texas, California, and Florida", so certainly nothing to scoff at). Doing a running average I also found the same.

As for Dst, we find more or less the same if we look at storms going beyond -400 nT, except for the earlier ones as it only goes back to 1957:

Quote

['1957-09-13', '10:00:00.000', '256', '-427.00']
['1958-02-11', '11:00:00.000', '042', '-426.00']
['1959-07-15', '19:00:00.000', '196', '-429.00']
['1989-03-14', '01:00:00.000', '073', '-589.00']
['2003-11-20', '21:00:00.000', '324', '-422.00']

These are just the most extreme storms though, there are of course plenty of other strong ones too, like the ones in 2001.

1 hour ago, Vancanneyt Sander said:

Well I do have to correct 😉 second strongest is also in 2001 (March 31st) with Bt maxed at 72nT (Bz min -47,41nT). Third place, also 2001 (November 24th) Bt maxed at 70,81 (Bz min -49,29). 
your date is at 11th place in the top 10 since 1996.

Maybe I wasn't so clear, but I was referring to the OMNIWeb data there. As I mentioned briefly in passing, they are hourly averages, and don't measure with the higher resolution you have in your archive; it was more to give a glimpse into older data too, since it goes all the way back to 1963. For e.g. the March 31 of 2001 event the maximum hourly average Bt listed there is just 48.2 nT, and the maximum Bz a few hours later listed at -44.7 nT; the November 24 of 2001 event is listed at a max Bt of 50.1 nT and a max Bz also a few hours later at -22.7. As I also mentioned I don't know how reliable that data is compared to more recent measurements, and there's no higher resolution there than hourly averages.

Edited April 26 by Philalethes
typo, Dst limit, duplicates