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Laboratory for Atmospheric and Space Physics

Extremely quiet Solar Wind conditions

September 24, 2012

On 11 May 1999, the solar wind density (n) upstream of the Earth dropped down to less than 1.0 particle per cubic centimeter. In fact, for the latter portion of the day the solar wind density was less than (or near) 0.1 cm-3 at the same time that the solar wind speed (V) was less than 300 km/s. This period has been characterized as “The Day the Solar Wind Ran Out of Gas”. The solar wind density and velocity combine to form the “dynamic pressure” (nmV2, where m is the ion mass) that compresses and confines the Earth’s magnetic field to form the magnetosphere. With a density perhaps one-fiftieth of normal and a solar wind speed perhaps half of normal, the dynamic pressure confining the magnetosphere was probably less than one part in 200 of its normal or typical value. Under such circumstances, the magnetosphere should have greatly expanded from its usual confined volume. And, in fact, observations suggest that this was true with the magnetospheric boundaries expanding out to (perhaps) near lunar distances (50 RE).

We have been very interested in how the Earth’s radiation belts – which are deeply embedded in the inner magnetosphere – would respond to this “free expansion” scenario. We have looked at SAMPEX, POLAR, GOES, GPS, and many other spacecraft which measure aspects of the Van Allen radiation belts. We find clear evidence that the radiation belts became much more azimuthally symmetric as the solar wind pressure decreased – as might have been expected – but we also find a subsequent, long-lasting, and really quite profound depletion of the radiation belts. Is this coincidence? Or was this long-lasting depletion caused somehow by the nearly absent solar wind? This is a puzzle that remains to be solved.

SAMPEX and POLAR data illustrate the situation in the accompanying figure. The top panels show daily “L-value” versus time plots for SAMPEX (uppermost panel) and for POLAR. Both of these are measured intensities of E > 2 MeV electrons where the fluxes are color-coded according to the color bar to the right. The data run from Day of Year 90 (31 March) to DOY 160 (9 June). Both SAMPEX and POLAR show a very substantial decrease in the radiation belt fluxes shortly after DOY 130 or so which then persists for the rest of the plot interval.

The bottom panel of the figure shows SAMPEX data at much higher resolution: These are “orbit-by-orbit” data. Here we see a temporary depletion of the particle flux at SAMPEX (low altitude) on Day 131 (11 May) (when the solar wind was also very low in density). Then on day 132, the electron fluxes came back in force. However, very early on Day 133 (13 May) the electrons suddenly dropped out again and they stayed depleted for the duration.

What caused this puzzling absence of the electrons about one day after the solar wind “disappearance” events? Were the two sets of events causally related, or just coincidental? We are actively working to answer these fascinating questions. Whatever the explanation, the electron fluxes in the radiation belt were extraordinarily low for many weeks after this period in mid-May. In fact, the electron fluxes did not attain high levels again until July or August of 1999. Further study should help us tie all of the observations together and give us a clearer understanding of how and why electron fluxes vary as they do in the radiation belts.

Contributed by D.N. Baker and SAMPEX/POLAR Team

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