My dissertation looks at how electric currents close in the upper
atmosphere, begging the question: Where do these currents come from? This video, created by the
Conceptual Image Lab at NASA's Goddard Space Flight Center, goes a long way in explaining it.
The Sun is constantly expelling ionized gas, or plasma, in all directions. Some of this plasma hits Earth,
which, if it weren't for our own terrestrial magnetic field, would strip away our atmosphere, just as it did
with Mars. Luckily, our magnetic field acts as a shield to this solar wind; a shield which sometimes breaks. We
call this breaking magnetic reconnection.
When this happens, the Earth's magnetic field attaches itself to the solar wind, which pulls it back toward the
nightside. There, the magnetic field lines, which can be thought of as elastic bands, pile up in what is called
the magnetotail. But only so many field lines can squeeze together before they too, just like on the dayside,
break and reconnect.
Immediately after this nightside reconnection occurs, since the field lines are under a good amount of tension,
they snap toward the Earth, carrying with them plenty of plasma. This plasma contains charged particles flying
past each other along the field lines, substantiating what is known as field-aligned electric current.
The Dungey Cycle
The above process is part of the Dungey Cycle, named after James Dungey for his 1961
paper. From the
nightside reconnection onward, the cycle continues whereupon these newly formed field lines accumulate around
local midnight. But, like in the magnetotail, the field lines prefer not to bunch up—they push away from each
other toward the dawn and dusk sides, ultimately replenishing the dayside magnetic field, where the cycle
repeats.
Margaret Kivelson and Christopher Russell, in chapter 9 of their 1995
textbook, provide
an excellent illustration, shown here, summarizing the Dungey cycle. They also illustrate how these
magnetospheric dynamics map to flows on the Earth's polar cap. Following along: (1) the dayside reconnection,
(2-5) the draping of the broken field lines to the magnetotail, (6) the nightside reconnection, (7-8) the
field lines snapping toward Earth while simultaneously lowering in latitude, and (9) the dayside replenishment.
On the Earth's polar caps, this looks like two symmetric D-shaped convection flow patterns. Plasma flows from
noon to midnight across the polar cap where it splits toward dawn and dusk and flows toward lower latitudes,
ultimately coming back around to the dayside.
Field-Aligned Current Closure
We care about these plasma flows because, as plasma moves forward in the northern polar cap, it
carries an electric field with it, directed to its right—an electric field along which currents can flow. We
call these Pedersen currents, shown here in a figure by Le et al. from their 2009
paper,
which illustrates how the Pedersen currents can connect the field-aligned currents to each other.
These electric fields, however, are not enough to sustain Pedersen currents. Collisions between our neutral
atmosphere and the charged particles subsiding in the plasmas allow these currents to flow perpendicular to the
Earth's magnetic field lines. This is why our atmosphere is such a crucial part of this system.
Now, these same collisions also give rise to a second type of current called the Hall current, which, perhaps
counterintuitively, runs opposite the flow of plasma. It also exists at a lower altitude than the Pedersen
currents and seemingly, according to this figure, are completely disconnected from the field-aligned currents.
My research challenges this ideal perspective of atmospheric current closure and looks at how the aurora, or
Northern and Southern lights, can impact this picture using 3D atmospheric plasma modeling.
Auroral Current Closure
The aurorae are more than just a stunning sight in the night sky; they are at the visible end of a complex
system affecting the aforementioned currents. The charged particles that rain down into the atmosphere
alongside the field-aligned currents can alter the plasma density and thus the collisionality. This, in turn,
impacts the efficiency of the Pedersen and Hall currents, creating a complicated, non-linear, yet
self-consistent system.
To illustrate the complexity of it all, this figure depicts the field-aligned current closure surrounding a
relatively simple set of auroral arcs. Each tube is what I call a current flux tube, in that it carries a single
amperage. Of the three, only the red one partially connects the field-aligned current sheets within this
simulation volume. In contrast to the above perspective, all three tubes connect the field-aligned current to
the Hall current in various ways.
A simulation like this one is made possible through the use of multiple scientific instruments. the map of
field-aligned current that this simulation is tasked with closing is plotted at the bottom. This map is
generated from field-aligned current data provided by the European Space Agency's
Swarm spacecraft. Furthermore, on
the eastern wall, a central cut of the plasma density resulting from the double-arc system is shown. This is
made possible through the use of multi-colored, all-sky auroral imagery provided by the Poker Flat Digital
All-sky Cameras (DASC) and additional computer
modeling. The light-blue arrows show the electric field as calculated by
GEMINI, but influenced by a global network of radars called
SuperDARN, or by the Poker Flat Incoherent Scatter Radar
(PFISR). Simulations such as these are
very much a group effort.
I encourage you to have a look at my dissertation, or our
2024/2025
papers on this subject!
