Ionospheric Plasma Flows Associated with the Formation of the Distorted Nightside End of A Transpolar Arc

We investigate ionospheric flow patterns from 28th January 2002 associated with the development of the nightside distorted end of a “J”-shaped Transpolar Arc (nightside distorted TPA). Based on the nightside ionospheric flows near to the TPA, detected by the SuperDARN radars, we discuss how the distortion of the nightside end toward the pre-midnight sector is produced. The “J”-shaped TPA was seen under southward Interplanetary Magnetic Field (IMF) conditions, in the presence of a dominant dawnward IMF-By component. At the onset time of the nightside distorted TPA, particular equatorward plasma flows at the TPA growth point were observed in the post-midnight sector, flowing out of the polar cap and then turning toward the pre-midnight sector of the main auroral oval along the distorted nightside part of the TPA. We suggest that these plasma flows play a key role in causing the nightside distortion of the TPA. SuperDARN also found ionospheric flows typically associated with “Tail Reconnection during IMF Northward Non-substorm Intervals” (TRINNIs) on the nightside main auroral oval before and during the TPA interval, indicating that nightside magnetic reconnection is an integral process to the formation of the nightside distorted TPA. During the TPA growth, SuperDARN also detected anti-sunward flows across the open-closed field line boundary on the dayside that indicate the occurrence of low-latitude dayside reconnection and ongoing Dungey cycle driving. This suggests that nightside distorted TPA can grow even in Dungey-cycle-driven plasma flow patterns.

On the accuracy of aurora visible boundaries in the OVATION Prime (PC) model

Оценка положения экваториальных границ аврорального овала при разных магнитосферных условиях, несёт в себе информацию о формирующихся плазменных структурах, глубине их проникновения во внутреннюю магнитосферу, движении внутренней границы плазменного слоя и т.д. Развитие алгоритмов определения положения видимой экваториальной границы аврорального овала является важной частью исследований, связанных с разработкой моделей химического состава ионосферы, моделей авроральных высыпаний частиц и оценки точности этих моделей. Немаловажную роль исследования полярных сияний (прогноз, интенсивность, положение) играют и для развития туристического сегмента в Арктике и информационных ресурсов служб мониторинга и прогноза космической погоды. В рамках исследования оценки точности положения видимых границ овала сияний в моделях авроральных высыпаний частиц была выбрана наземная наблюдательная сеть оптических камер всего неба проекта THEMIS, запущенная в 2008 г., и модифицированная модель OVATION Prime (PC), разработанная в отделе Геофизики ФГБУ «ААНИИ использующая в качестве входного параметра наземный индекс полярной шапки (PC-индекс). The location of the equatorial boundaries of the auroral oval under different magnetospheric conditions contains information about the forming plasma structures, the depth of their penetration into the inner magnetosphere, the motion of the inner boundary of the plasma layer, etc. The development of methods and algorithms for determining the position of the visible equatorial boundary of the auroral oval is an important part of research related to the development of models of the chemical composition of the ionosphere, models of auroral particle precipitation, and assessment of the accuracy of these models. Research of aurora borealis (forecast, intensity, position) also plays an important role for the development of the tourist segment in the Arctic and information resources of space weather monitoring and forecasting services.

Ground-Based GNSS and Satellite Observations of Auroral Ionospheric Irregularities during Geomagnetic Disturbances in August 2018

The 25–26 August 2018 space weather event occurred during the solar minimum period and surprisingly became the third largest geomagnetic storm of the entire 24th solar cycle. We analyzed the ionospheric response at high latitudes of both hemispheres using multi-site ground-based GNSS observations and measurements onboard Swarm and DMSP satellites. With the storm development, the zones of intense ionospheric irregularities of auroral origin largely expanded in size and moved equatorward towards midlatitudes as far as ~55–60° magnetic latitude (MLAT) in the American, European, and Australian longitudinal sectors. The main ionospheric trough, associated with the equatorward side of the auroral oval, shifted as far equatorward as 45–50° MLAT at both hemispheres. The interhemispheric comparison revealed a high degree of similarity in a large expansion of the auroral irregularities oval towards midlatitudes, in addition to asymmetrical differences in terms of larger intensity of plasma density gradients and structures over the Southern auroral and polar cap regions. Evolution of the intense ionospheric irregularities and equatorward expansion of the auroral irregularities oval were well correlated with increases of geomagnetic activity and peaks of the auroral electrojet index.

Direct Connection Between Auroral Oval Streamers/Flow Channels and Equatorward Traveling Ionospheric Disturbances

We use simultaneous auroral imaging, radar flows, and total electron content (TEC) measurements over Alaska to examine whether there is a direct connection of large-scale traveling ionospheric disturbances (LSTIDs) to auroral streamers and associated flow channels having significant ground magnetic decreases. Observations from seven nights with clearly observable flow channels and/or auroral streamers were selected for analysis. Auroral observations allow identification of streamers, and TEC observations detect ionization enhancements associated with streamer electron precipitation. Radar observations allow direct detection of flow channels. The TEC observations show direct connection of streamers to TIDs propagating equatorward from the equatorward boundary of the auroral oval. The TIDs are also distinguished from the streamers to which they connect by their wave-like TEC fluctuations moving more slowly equatorward than the TEC enhancements from streamer electron precipitation. TIDs previously observed propagating equatorward from the auroral oval have been identified as LSTIDs. Thus, the TIDs here are likely LSTIDs, but we lack sufficient TEC coverage necessary to demonstrate that they are indeed large scale. Furthermore, each of our events shows TID’s connection to groups of a few streamers and flow channels over a period in the order of 15 min and a longitude range of ∼15–20°, and not to single streamers. (Groups of streamers are common during substorms. However, it is not currently known if streamers and associated flow channels typically occur in such groups.) We also find evidence that a flow channel must lead to a sufficiently large ionospheric current for it to lead to a detectable LSTID, with a few tens of nT ground magnetic field decreases not being sufficient.

Two-Dimensional Structure of Flow Channels and Associated Upward Field-Aligned Currents: Model and Observations

Flow bursts are a major component of transport within the plasma sheet and auroral oval (where they are referred to as flow channels), and lead to a variety of geomagnetic disturbances as they approach the inner plasma sheet (equatorward portion of the auroral oval). However, their two-dimensional structure as they approach the inner plasma sheet has received only limited attention. We have examined this structure using both the Rice Convection Model (RCM) and ground-based radar and all sky imager observations. As a result of the energy dependent magnetic drift, the low entropy plasma of a flow burst spreads azimuthally within the inner plasma sheet yielding specific predictions of subauroral polarization stream (SAPS) and dawnside auroral polarization stream (DAPS) enhancements that are related to the field-aligned currents associated with the flow channel. Flow channels approximately centered between the dawn and dusk large-scale convection cells are predicted to give significant enhancements of both SAPS and DAPS, whereas flow channel further toward the dusk (dawn) convection cell show a far more significant enhancement of SAPS (DAPS) than for DAPS (SAPS). We present observations for cases having good coverage of flow channels as they approach the equatorward portion of the auroral oval and find very good qualitative agreement with the above RCM predictions, including the predicted differences with respect to flow burst location. Despite there being an infinite variety of flow channels’ plasma parameters and of background plasma sheet and auroral oval conditions, the observations show the general trends predicted by the RCM simulations with the idealized parameters. This supports that RCM predictions of the azimuthal spread of a low-entropy plasma sheet plasma and its associated FAC and flow responses give a realistic physical description of the structure of plasma sheet flow bursts (auroral oval flow channels) as they reach the inner plasma sheet (near the equatorward edge of the auroral oval).

Magnetosphere–Ionosphere Coupling

The process of magnetosphere-ionosphere coupling involves the transport of vast quantities of energy and momentum between a magnetized planet and its space environment, or magnetosphere. This transport involves extended, global sheets of electrical current, which flows along magnetic field lines. Some of the charged particles, which carry this current rain down onto the planet’s upper atmosphere and excite aurorae–beautiful displays of light close to the magnetic poles, which are an important signature of the physics of the coupling process. The Earth, Jupiter, and Saturn all have magnetospheres, but the detailed physical origin of their auroral emissions differs from planet to planet. The Earth’s aurora is principally driven by the interaction of its magnetosphere with the upstream solar wind—a flow of plasma continually emanating from the Sun. This interaction imposes a particular pattern of flow on the plasma within the magnetosphere, which in turn determines the morphology and intensity of the currents and aurorae. Jupiter, on the other hand, is a giant rapid rotator, whose main auroral oval is thought to arise from the transport of angular momentum between the upper atmosphere and the rotating, disc-like plasma in the magnetosphere. Saturn exhibits auroral behavior consistent with a solar wind–related mechanism, but there is also regular variability in Saturn’s auroral emissions, which is consistent with rotating current systems that transport energy between the magnetospheric plasma and localized vortices of flow in the upper atmosphere/ionosphere.

Geomagnetic method for automatic diagnostics of auroral oval boundaries in two hemispheres of Earth

The ground-based automatic method for determining auroral oval (AO) boundaries developed by the authors [Lunyushkin, Penskikh, 2019] has been modified and expanded to the Southern Hemisphere. Input data of the method contains large-scale distributions of the equivalent current function and field-aligned current density calculated in the polar ionospheres of two hemispheres in a uniform ionospheric conductance approximation based on the magnetogram inversion technique and the geomagnetic database of the world network of stations of the SuperMAG project. The software implementation of the method processes large volumes of time series of input data and produces coordinates of the main boundaries of AO in both hemi- spheres: the boundaries of the ionospheric convection reversal, the AO polar and equatorial boundaries, the lines of maximum density of field-aligned currents and auroral electrojets. The automatic method reduces the processing time for a given amount of data by 2–3 orders of magnitude (up to minutes and hours) compared to the manual method, which requires weeks and months of laborious operator work on the same task, while both methods are comparable in accuracy. The automatic geomagnetic method has been tested for diagnostics of AO boundaries during the isolated substorm of August 27, 2001, for which the expected synchronous dynamics of polar caps in two hemispheres has been confirmed. We also show the AO boundaries identified are in qualitative agreement with simultaneous AO images from the IMAGE satellite, as well as with the results of the OVATION and APM models; the boundary of ionospheric convection reversal, determined by the geomagnetic method in two hemispheres, is consistent with the maps of the electric potential of the ionosphere according to the SuperDARN-RG96 model.

Geomagnetic method for automatic diagnostics of auroral oval boundaries in two hemispheres of Earth

The ground-based automatic method for determining auroral oval (AO) boundaries developed by the authors [Lunyushkin, Penskikh, 2019] has been modified and expanded to the Southern Hemisphere. Input data of the method contains large-scale distributions of the equivalent current function and field-aligned current density calculated in the polar ionospheres of two hemispheres in a uniform ionospheric conductance approximation based on the magnetogram inversion technique and the geomagnetic database of the world network of stations of the SuperMAG project. The software implementation of the method processes large volumes of time series of input data and produces coordinates of the main boundaries of AO in both hemi- spheres: the boundaries of the ionospheric convection reversal, the AO polar and equatorial boundaries, the lines of maximum density of field-aligned currents and auroral electrojets. The automatic method reduces the processing time for a given amount of data by 2–3 orders of magnitude (up to minutes and hours) compared to the manual method, which requires weeks and months of laborious operator work on the same task, while both methods are comparable in accuracy. The automatic geomagnetic method has been tested for diagnostics of AO boundaries during the isolated substorm of August 27, 2001, for which the expected synchronous dynamics of polar caps in two hemispheres has been confirmed. We also show the AO boundaries identified are in qualitative agreement with simultaneous AO images from the IMAGE satellite, as well as with the results of the OVATION and APM models; the boundary of ionospheric convection reversal, determined by the geomagnetic method in two hemispheres, is consistent with the maps of the electric potential of the ionosphere according to the SuperDARN-RG96 model.