160 research outputs found
Thermospheric nitric oxide at higher latitudes: Model calculations with auroral energy input
No full text
Comparisons of electron energy deposition derived from observations of lower thermospheric nitric oxide and from X-ray bremsstrahlung measurements
No full textCorrection to “Height‐integrated conductivity in auroral substorms, 1, Data” by J. W. Gjerloev and R. A. Hoffman
No full textA new technique for determining Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE)
No full textWe present a new quantitative technique that determines the times and durations of substorm expansion and recovery phases and possible growth phases based on percentiles of the rate of change of auroral electrojet indices. By being able to prescribe different percentile values, we can determine the onset and duration of substorm phases for smaller or larger variations of the auroral index or indeed any auroral zone ground-based magnetometer data. We apply this technique to the SuperMAG AL (SML) index and compare our expansion phase onset times with previous lists of substorm onsets. We find that more than 50% of events in previous lists occur within 20 min of our identified onsets. We also present a comparison of superposed epoch analyses of SML based on our onsets identified by our technique and existing onset lists and find that the general characteristics of the substorm bay are comparable. By prescribing user-defined thresholds, this automated, quantitative technique represents an improvement over any visual identification of substorm onsets or indeed any fixed threshold method
laurenorr/substorm-community: Nature Communications 2021
No full textCode used within "Network Community Structure of Substorms using SuperMAG Magnetometers" by L. Orr, S. C. Chapman, J. W. Gjerloev, and W. Gu
Are there optical differences between storm-time substorms and isolated substorms?
No full textWe have performed an extensive analysis of auroral optical events
(substorms) that occurred during the development of the main phase of
magnetic storms. Using images from the Earth Camera on the Polar spacecraft
(Frank et al., 1995), we compared the optical emission features of substorms
occurring during 16 expansion phases of magnetic storms with the features of
isolated substorms occurring during non-storm times. The comparison used two
techniques, visual inspection and statistical comparisons. The comparisons
were based on the common characteristics seen in isolated substorms that
were initially identified by Akasofu (1964) and quantified by Gjerloev et
al. (2008). We find that when auroral activity does occur during main phase
development the characteristics of the aurora are very dissimilar to those
of the classical isolated substorm. The primary differences include the lack
of a surge/bulge, lack of bifurcation of the aurora, much shorter expansion
phases, and greater intensities.
Since a surge/bulge and bifurcation of the aurora are characteristics of the
existence of a substorm current wedge, a key component of the
magnetosphere-ionosphere current system during substorms, the lack of this
component would indicate that the classical substorm model does not apply to
the storm time magnetosphere-ionosphere current system. Rather several of
the analyses suggest that the storm-time substorms are associated more
closely with the auroral oval, at least spatially, and, therefore, probably
with the plasma sheet dynamics during the main phase development. These
results then must call into question the widely held assumption that there
is no intrinsic difference between storm-time substorms and classical
isolated substorms
Substorm behavior of the auroral electrojet indices
No full textAbstract. The behavior of the auroral electrojet indices AU and AL during classical substorms is investigated by the use of global auroral images. A superposition of the 12 AE stations onto global auroral images and identification of the AL and AU contributing stations enable an understanding of the temporal as well as spatial behavior of the indices with respect to the substorm coordinate system and timeframe. Based on this simple technique it was found that at substorm onset the AL contributing station makes a characteristic jump from a location near the dawn terminator to the onset region, typically bypassing one or more AE stations. During the expansion phase this station typically lies at the poleward edge of the surge region. This is the location of the intense substorm current wedge electrojet in the semiempirical self-consistent substorm model of the three-dimensional current system by Gjerloev and Hoffman (2002). This current wedge is fed primarily pre-midnight by an imbalance of the Region 0 and Region 1 field-aligned currents, not from the dawnside westward electrojet. Then during the early recovery phase the AL contributing station jumps back to the dawn sector. The defining AU station does not show any similar systematic behavior. We also find that the dawn side westward electrojet seems to be unaffected by the introduction of the substorm current wedge. According to our model, much of this current is closed to the magnetosphere as it approaches midnight from dawn. Based on the characteristics of the AL station jumps, the behavior of the dawn-side electrojet, and the understanding of the three-dimensional substorm current system from our model, we provide additional experimental evidence for, and an understanding of, the concept of the two component westward electrojet, as suggested by Kamide and Kokubun (1996).
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The convection electric field in auroral substorms
No full textDynamics Explorer 2 (DE 2) electric field and ion drift data are used in a statistical study of the ionospheric convection electric field in bulge-type auroral substorms. Thirty-one individual DE 2 substorm crossings were carefully selected and organized by the use of global auroral images obtained by DE 1. The selected passes, which occurred during substorm expansion phase, maximum, or early recovery phase, cover the entire nighttime substorm. The organization of the data used the method developed by Fujii et al. [1994], which divided the data into six local time sectors covering the nighttime substorm region. Following the procedures employed in the paper by Gjerloev and Hoffman [2000b], the latitudinal width and location of each auroral oval crossing was then adjusted to fit the sector average. In addition to the detailed study of the characteristics of the field within each sector this database enabled us to compile a model of the ionospheric convection electric field. The characteristics of the premidnight convection reversal show a pronounced local time dependency. Far west of the surge it is a fairly well defined point reversal or convection shear. Approaching the surge and within the surge it is a region of weak electric fields increasing in width toward midnight that separates regions of equatorward and poleward electric fields. Therefore we adopt the term Harang region rather than the Harang discontinuity for the premidnight convection reversal. A relatively narrow convection channel is coincident with the highest conductances located just poleward of the Harang region. This channel drives the substorm current wedge component of the westward electrojet in the surge and middle surge sectors. It is present in all premidnight passes and consequently is an integral part of the three-dimensional substorm current wedge system
Substorm behavior of the auroral electrojet indices
No full textInternational audienceThe behavior of the auroral electrojet indices AU and AL during classical substorms is investigated by the use of global auroral images. A superposition of the 12 AE stations onto global auroral images and identification of the AL and AU contributing stations enable an understanding of the temporal as well as spatial behavior of the indices with respect to the substorm coordinate system and timeframe. Based on this simple technique it was found that at substorm onset the AL contributing station makes a characteristic jump from a location near the dawn terminator to the onset region, typically bypassing one or more AE stations. During the expansion phase this station typically lies at the poleward edge of the surge region. This is the location of the intense substorm current wedge electrojet in the semiempirical self-consistent substorm model of the three-dimensional current system by Gjerloev and Hoffman (2002). This current wedge is fed primarily pre-midnight by an imbalance of the Region 0 and Region 1 field-aligned currents, not from the dawnside westward electrojet. Then during the early recovery phase the AL contributing station jumps back to the dawn sector. The defining AU station does not show any similar systematic behavior. We also find that the dawn side westward electrojet seems to be unaffected by the introduction of the substorm current wedge. According to our model, much of this current is closed to the magnetosphere as it approaches midnight from dawn. Based on the characteristics of the AL station jumps, the behavior of the dawn-side electrojet, and the understanding of the three-dimensional substorm current system from our model, we provide additional experimental evidence for, and an understanding of, the concept of the two component westward electrojet, as suggested by Kamide and Kokubun (1996)
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