Abstract. It is well known that the polar cap, delineated by the open–closed field line boundary (OCB),
responds to changes in the interplanetary magnetic field (IMF).
In general, the boundary moves equatorward when the IMF turns southward and contracts
poleward when the IMF turns northward. However,
observations of the OCB are spotty and limited in local time,
making more detailed studies of its IMF dependence difficult.
Here, we simulate five solar storm periods with the coupled model consisting of the Open
Geospace General Circulation Model (OpenGGCM) coupled with the Coupled Thermosphere Ionosphere
Model (CTIM) and the Rice Convection Model (RCM),
i.e., the OpenGGCM-CTIM-RCM, to estimate the location and dynamics of the OCB.
For these events, polar cap boundary location observations are also obtained from Defense Meteorological
Satellite Program (DMSP) precipitation spectrograms and compared with the model output.
There is a large scatter in the DMSP observations and in the model output.
Although the model does not predict the OCB with high fidelity for every observation,
it does reproduce the general trend as a function of IMF clock angle.
On average, the model overestimates the latitude of the open–closed field line boundary
by 1.61∘. Additional analysis of the simulated polar cap boundary dynamics across
all local times shows that the MLT of the largest polar cap expansion closely correlates
with the IMF clock angle, that the strongest correlation occurs when the IMF is southward, that
during strong southward IMF the polar cap shifts sunward, and that the polar cap rapidly
contracts at all local times when the IMF turns northward.
We present results from a numerical study of small‐scale, intense magnetic field‐aligned currents observed in the vicinity of the discrete auroral arc by the Magnetosphere‐Ionosphere Coupling in the Alfvén Resonator (MICA) sounding rocket launched from Poker Flat, Alaska, on 19 February 2012. The goal of the MICA project was to investigate the hypothesis that such currents can be produced inside the ionospheric Alfvén resonator by the ionospheric feedback instability (IFI) driven by the system of large‐scale magnetic field‐aligned currents interacting with the ionosphere. The trajectory of the MICA rocket crossed two discrete auroral arcs and detected packages of intense, small‐scale currents at the edges of these arcs, in the most favorable location for the development of the ionospheric feedback instability, predicted by the IFI theory. Simulations of the reduced MHD model derived in the dipole magnetic field geometry with realistic background parameters confirm that IFI indeed generates small‐scale ULF waves inside the ionospheric Alfvén resonator with frequency, scale size, and amplitude showing a good, quantitative agreement with the observations. The comparison between numerical results and observations was performed by “flying” a virtual MICA rocket through the computational domain, and this comparison shows that, for example, the waves generated in the numerical model have frequencies in the range from 0.30 to 0.45 Hz, and the waves detected by the MICA rocket have frequencies in the range from 0.18 to 0.50 Hz.
We present results from the ionospheric heating experiment conducted at the High Frequency Active Auroral Research Program (HAARP) facility, Alaska, on 12 March 2013. During the experiment, HAARP transmitted in the direction of the magnetic zenith X‐mode 4.57‐MHz wave. The transmitted power was modulated with the frequency of 0.9 mHz, and it was pointed on a 20‐km spot at the altitude of 120 km. The heating (1) generates disturbances in the magnetic field detected with the fluxgate magnetometer on the ground and (2) produces bright luminous spots in the ionosphere, observed with the HAARP telescope. Numerical simulations of the 3‐D reduced magnetohydrodynamic (MHD) model reveal that these effects can be related to the magnetic field‐aligned currents, excited in the ionosphere by changing the conductivity in the E region when the large‐scale electric field exists in the heating region.
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