The characteristics of particle injections in the Earth’s inner magnetosphere question our traditional representation for the large-scale electric fields. We argue that including a localized source of electric fields known as the SAPS is the key.
The interaction between the Sun and the Earth’s magnetic field often transmits significant energy to the electrons and protons present in the magnetospheric tail, a region of space located far from the Earth’s night-side. As a result, these particles move closer to the Earth. They are “injected” into the Earth’s inner space environment.
Since the 1970s, scientists have theorized that electrons and protons with the same initial kinetic energy starting from the same radial location have about the same distance of closest approach to the Earth.
Yet, recent observations from the Van Allen Probes reveal that this is not the case!
Electrons appear to be systematically injected “deeper” than protons. So what happens along the way to make electrons approach Earth at closer distances?
Our latest research combines observations from both space and the ground. We show that a localized source of electric fields called a Sub-Auroral Polarization Stream (aka a “SAPS”) is always present during these injections. We argue that a SAPS acts like a marathon aid station for electrons in that it provides them with additional energy. As a result, electrons “move faster” and approach the Earth at closer distances.
New research published in Geophysical Research Letters extends this, analyzing the effect of space weather on plasma transport. The findings are similar to what one would expect in on-the-ground traffic: the worse the weather conditions, the slower the traffic flows (in space, this is the case below an altitude of about 13 000 km = 8 000 miles).
Yet, there is no rain or snow in space … so how does weather manifest?!
The Sun constantly emits both particles and energy, but it can be the scene of violent phenomena. While most of the particles and energy emitted by the Sun and flowing towards Earth are deflected by the Earth’s magnetic field, some of them can be transmitted to Near-Earth space.
On the plus side: this can lead to the appearance of magnificent auroras.
On the minus side: this can pose a hazard for human activities, because space-borne and ground-based technological systems can be affected.
Just like “weather” refers to the state of the Earth’s atmosphere, the term “space weather” is the used to describe conditions in space. And just like “weather” conditions affect traffic flows of cars and trucks, we expected that “space weather” would affect the flow of ions and electrons in space, although we were not sure how. For the first time, we could answer that question.
It is often assumed that the Sun directly affects near-Earth space. Our study highlights the important role of go-between played by the ionosphere. Our findings offer new context to existing theories and they provide a starting point to better monitoring space weather.
I am thrilled to be the co-author of Juan Roederer’s latest commentary entitled “Coordinates for representing radiation belt flux”. In a nutshell, the objective of this work is to briefly review the history of radiation belt parametrization, to present some “recipes” on how to compute adiabatic parameters (you may be familiar with L*… but have you ever heard of E*? and what about α*?), and to apply these recipes to a real event in which magnetospheric disturbances adiabatically affect the particle fluxes measured onboard the Van Allen Probes.
Juan G. Roederer is one of the founding fathers of radiation belt science. In the community, he is known primarily (1) for his classic textbook on radiation belt dynamics (Dynamics of Geomagnetically Trapped Radiation, 1970) and (2) for the coordinate that he introduced 50 years ago, namely “L-star”, a.k.a. “Roederer-L”.
One might wonder why I am involved in this work.
In fact, the story is pretty uplifting, so let me tell you how I became Roederer’s coauthor.
During my first year as a phD student in Toulouse, France, my supervisor introduced me to adiabatic theory and I dove into the classics: Northrop, Schulz and Lanzerotti, and (of course!) Roederer, whose book happened to be my supervisor’s favorite. I had precise questions on some aspects of Roederer’s book which I brought to my supervisor. He said I should try asking Roederer directly. “Try sending him an email. Worst case scenario: he won’t answer”.
I spent half a day with Google Translate, trying to formulate the most polite and accurate email in order to maximize my chances for a response.
I got a response!
But the first answer I got was not quite what I had expected. Juan said that I had caught him at the wrong time, that he was giving an organ recital in Göttigen, Germany, and that later he would be traveling to China. He also wrote that he would answer my questions in a month or so. I was a bit confused given that he was supposed to be an 80 year-old scientist living in Fairbanks, Alaska. How could he be more active than me?
About a month later, he did indeed answer my science questions. We started discussing some details of his book, and in particular the time variations of L* (the building block of “radial diffusion”). As a result of these exchanges, Juan invited me to review one appendix of the new edition of his classic textbook. Next thing I knew, he sent me another appendix, then a chapter, and finally he sent me the whole draft version of the book (twice!).
In 2014, we finally met in real life at my first AGU conference in San Francisco, California. I had defended my thesis and was a postdoc. It was more than three years after our first email exchange, and a few months after the publication of the 2nd edition of the Dynamics of Magnetically Trapped Particles. Needless to say, I had received my autographed copy.
One of my ambitions at the time was to co-author an article with Juan. We were both intrigued by these “zebra stripe” observations that were being advertised by the Van Allen Probe mission. So we met and worked on that topic for a week. Juan made drawings on a paper board, and I elaborated on the equations after work. A few months later, our paper was published. (and yes, I have also a post on the zebra stripes).
The commentary that we have just published is the continuation of our collaboration.
For this work, I was in charge of the computations, figures, and editing.
First, Juan tells the history of radiation belt parametrization. The differences between “distance to the equatorial point of a field line”, McIlwain’s L-value, and the trapped particle’s adiabatic L*parameter are explained (this can be pretty confusing given that these parameters all merge in a dipole magnetic field!). Then, we present a readjustment on adiabatic theory, and we explain how to reformulate measurements in terms of L*, E*, α* (and j*!). Finally, we illustrate the method on a real event.
It is our hope that this work will send out an “adiabatic wake-up call” to the community, for it demonstrates the importance of a rigorous use of adiabatic theory in order to deliver higher quality research. We are looking forward to your feedback!
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!
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.
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.
We found that the electric fields lead to a rotational angular speed 10% smaller than the rotational angular speed of the Earth.
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.
“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:
“Zebra stripes” are signature of an azimuthal dependence in trapped particle distributions below L ~ 3.
High-altitude ionospheric winds affect the azimuthal distribution of radiation belt intensity.
The number of stripes indicates how many hours the population spent drifting under quiet conditions.
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: