First Full Mapping of Near-Earth Plasma Transport Achieved, Thanks to the Van Allen Probes

Maps are developed to best describe what surrounds us. That is true on the ground, and it is also true in space. There, the most interesting maps are maps of traffic conditions, i.e., maps that tell us how fast transport is, and in what the direction the flow is, depending on location. The only slight difference is that in space we are dealing with plasma transport, ions and electrons, rather than flows of cars and trucks.

To detail traffic in space, we must know both the magnetic field and the electric field: how strong are they? In what direction are they pointing? But unlike the magnetic field, the electric field is very difficult to measure, especially close to Earth! To circumvent this challenge, scientists have made assumptions and used theoretical considerations, rather than observations, to draw a simple picture of what they think the electric field should be around Earth.

This simple picture implies that the traffic conditions are as if the cold plasma was riding a giant merry-go-round, with the Earth in the center. In other words, the cold plasma is thought to rotate at the same speed as the Earth’s rotational speed, i.e., to be in corotation with the planet. Yet, this has not been proven experimentally. In fact, some sporadic particle measurements have already indicated that the merry-go-round picture was not quite right.

Using data from the Van Allen Probe satellites, we managed to make the first ever comprehensive observations of plasma transport due to the electric field close to Earth. This is a technical feat that allows us to test our 50 year old theories, at last! And the results are exciting!

Based on an analysis of more than 2 years of data, we confirmed that the cold plasma was not simply riding along a merry-go-round. In particular, we found that the speed and direction at which the plasma was drifting depended on the time of the day (or, in other words, that they depended on the location with respect to the Sun). We also found that, on average, the rotational speed of the cold plasma was 5 to 10% slower than that of the Earth. We must now understand why!

In short, our observations offer new context to existing theories; theories that merit reviews in our ongoing quest to better understand near Earth space!

 

For more information on that topic, you can:

  • Access the article [here]
  • Access the article by Forrest Mozer on electric field measurements in space [here]
  • Download my invited presentation @ the AGU fall meeting 2016 [here] or simply scroll through it:

The Van Allen Probes Deliver Promising Measurements of the Electric Drift in the Inner Belt

Electric fields play a fundamental role in space physics. Yet, experimentally, they are difficult to observe. Closest to Earth, in the inner belt and slot region, electric field measurements are impeded by spacecraft motion, for instance. When a spacecraft passes through perigee, it does so very fast, travelling tens of kilometers per second. As a result, the sensors inevitably detect a large motional electric field; but in reality this is an illusion. For the Van Allen Probes, the motional electric field constitutes at least 95% of the electric fields measured close to perigee. A measurement accuracy much better than 95% is therefore required to observe the electric field in the inner magnetosphere. In an article which has just been published in Geophysical Research Letters, it is shown that the Van Allen Probes achieve such accuracy.

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Together with Pr. Forrest Mozer, we present an analysis of two years of electric and magnetic measurement at one altitude chosen to enable comparisons with ground observations (L=1.4). The measurements of electric drift (ExB/B2) revealed departures from the traditional motion of corotation with the Earth in 2 ways.

  1. We found that the electric fields lead to a rotational angular speed 10% smaller than the rotational angular speed of the Earth.
  2. We detected the ionosphere dynamo electric fields, which lead to a magnetic local time dependence of the electric fields, in both radial and azimuthal directions.

Such pieces of information are important when discussing, for instance, the structure and dynamics of the plasmasphere or the radial transport of trapped particles at lowest altitudes.

The story is to be continued!

 

For more information on that topic, you can:

    • Access the article [here]
    • Download my poster from the GEM-CEDAR 2016 [here]
    • Download the GEM-CEDAR talk [here] or scroll through it :

Zebra Stripes in Outer Space!

“Zebra Stripes in Outer Space”:
Long-standing mystery of radiation belt signature explained

For some months, I had the privilege to work with Juan Roederer on a new theory to explain the existence of “zebra stripes” in the Earth’s Van Allen radiation belt particles, these zebra-like patterns in energy spectrograms that indicate highly organized compressions and expansions of the radiation belt particles as they circle the Earth. The result of our cogitation is the object of an article published in the Journal of Geophysical Research – Space Physics.

In 3 tweets, we argue that:
  1. “Zebra stripes” are signature of an azimuthal dependence in trapped particle distributions below L ~ 3.
  2. High-altitude ionospheric winds affect the azimuthal distribution of radiation belt intensity.
  3. The number of stripes indicates how many hours the population spent drifting under quiet conditions.
In particular….

The mechanism that we propose to explain such feature can be likened to what happens in multi-lane highway traffic during rush hour, when an initial group of evenly spaced cars is segregated into high/low density bunches according to their acceleration power and the varying speed limits along their route. In the case of the Van Allen belt particles, it is the upper atmosphere that acts as the “traffic regulator”: ionospheric zonal winds in equatorial regions, which during quiet solar activity conditions blow eastward around midnight and westward around noon, induce electric fields which speed up or slow down the drift of the particles around the planet.

The effect of a variation in speed on the density of cars (aka the particles!) is illustrated in the following video:

For more info and technical details: