An optimum solar wind coupling function for the <i>AL</i> index

McPherron, R. L.; Hsu, T.; Chu, Xiangning

doi:10.1002/2014ja020619

Cited by 59 publications

(67 citation statements)

References 82 publications

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“…While the equinoctial effect and the Russell‐McPherron effect contribute to the geomagnetic activity, it is clear that some other underlying mechanisms are required to explain the observed seasonal and diurnal variations. As shown in Figure and recent studies [ Newell and Gjerloev , ; Guo et al , ; McPherron et al , ], the solar wind driving evaluated using various solar wind coupling functions reveals a clear annual variation. The seasonal variations of the MPB index are not symmetric respective to summer solstice and are modulated by the solar wind driving in every single year.…”

Section: Thirty‐one Years Of Mpb Statisticsmentioning

confidence: 52%

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Solar cycle dependence of substorm occurrence and duration: Implications for onset

et al. 2015

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Magnetospheric substorms represent a major energy release process in Earth's magnetosphere. Their duration and intensity are coupled to solar wind input, but the precise way the solar wind energy is stored and then released is a matter of considerable debate. Part of the observational difficulty has been the gaps in the auroral electrojet index traditionally used to study substorm properties. In this study, we created a midlatitude positive bay (MPB) index to measure the strength of the substorm current wedge. Because this index is based on midlatitude magnetometer data that are available continuously over several decades, we can assemble a database of substorm onsets lasting 31 years (1982–2012). We confirmed that the MPB onsets have a good agreement (±2 min) with auroral onsets as determined by optical means on board the IMAGE mission and that the MPB signature of substorms is robust and independent of the stations' position relative to ionospheric currents. Using the MPB onset, expansion, and recovery as a proxy of the respective substorm quantities, we found that the solar cycle variation of substorm occurrence depends on solar wind conditions and has a most probable value of 80 min. In contrast, the durations of substorm expansion and recovery phases do not change with the solar cycle. This suggests that the frequency of energy unloading in the magnetosphere is controlled by solar wind conditions through dayside reconnection, but the unloading process related to flux pileup in the near‐Earth region is controlled by the magnetosphere and independent of external driving.

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Section: Thirty‐one Years Of Mpb Statisticsmentioning

confidence: 52%

“…Akasofu's

ε = v B^{2} {sin}^{4} ((), \frac{θ}{2}) l_{0}^{2}

[ Akasofu , ] is proportional to the solar wind speed. The universal coupling function

d \emptyset true/ d t = v^{\frac{4}{3}} B^{\frac{2}{3}} {sin}^{\frac{8}{3}} ((), \frac{θ}{2})

[ Newell et al , ] and the optimum coupling function

v^{1.74} B^{0.74} {sin}^{3.77} ((), \frac{θ}{2})

[ McPherron et al , ] also suggest that the solar wind input is directly related to solar wind speed.…”

Section: Discussionmentioning

confidence: 99%

Solar cycle dependence of substorm occurrence and duration: Implications for onset

et al. 2015

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“…We examine the seasonal variation of high‐latitude geomagnetic activity because high‐latitude geomagnetic activity better responds to the high‐speed solar wind streams (HSSs) originated from large solar coronal holes than geomagnetic activity at low latitude to midlatitude, where coronal mass ejections (CMEs) have a relatively larger role (Finch et al, ; Holappa, Mursula, Asikainen, & Richardson, ). Moreover, most studies of the seasonal variation of geomagnetic activity are based on low‐latitude to midlatitude observations and indices, and only relatively few studies (e.g., Cliver et al, ; Finch et al, ; McPherron, Hsu, & Chu, ; Mursula et al, ; Tanskanen et al, ) have examined the seasonal variation of geomagnetic activity at high latitudes.…”

Section: Introductionmentioning

confidence: 99%

Seasonal Variation of High‐Latitude Geomagnetic Activity in Individual Years

2017

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We study the seasonal variation of high‐latitude geomagnetic activity in individual years in 1966–2014 (solar cycles 20–24) by identifying the most active and the second most active season based on westward electrojet indices AL (1966–2014) and IL (1995–2014). The annual maximum is found at either equinox in two thirds and at either solstice in one third of the years examined. The traditional two‐equinox maximum pattern is found in roughly one fourth of the years. We found that the seasonal variation of high‐latitude geomagnetic activity closely follows the solar wind speed. While the mechanisms leading to the two‐equinox maxima pattern are in operation, the long‐term change of solar wind speed tends to mask the effect of these mechanisms for individual years. Large cycle‐to‐cycle variation is found in the seasonal pattern: equinox maxima are more common during cycles 21 and 22 than in cycles 23 or 24. Exceptionally long winter dominance in high‐latitude activity and solar wind speed is seen in the declining phase of cycle 23, after the appearance of the long‐lasting low‐latitude coronal hole.

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“…An example of the MPB index variation during substorm activity on 26 February 2008 is plotted in Figure . Figure a contains the optimum solar wind coupling function for AL (McPherron et al, ); Figure b is the SuperMag SML index; Figure c contains the Chu MPB index series; Figure d displays a standardized version of this series; Figure e is the power in Pi 2 pulsations at U.S. Geological Survey stations before 12 UT and at the Kakioka station after 12 UT. A vertical red line across Figure b at 14:32 UT is the only substorm onset listed in the SuperMag onset list for this day (Newell & Gjerloev, ).…”

Section: Calculation Of the Mpb Indexmentioning

confidence: 99%

“…In the right column the McPMPB list, the ChuMPB list, and the SuperMag lists were used to define epoch zero of the analysis. Figure d presents ensemble medians of the optimum AL solar wind coupling function (McPherron et al, ). The blue trace in all panels shows the superposed median for 44,943 McPMPB onsets (1982–2012).…”

Section: Properties Of Midlatitude Positive Bays and Associated Phenomentioning

confidence: 99%

The Midlatitude Positive Bay Index and the Statistics of Substorm Occurrence

McPherron

Chu

2018

JGR Space Physics

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The auroral breakup and expansion are essential features of a magnetospheric substorm. Negative bays beneath the aurora and positive bays at midlatitudes accompany the expansion. These effects are caused by the substorm current wedge. The negative bay strength is provided by the auroral electrojet lower (AL) index. At midlatitudes the positive bay is caused by the field‐aligned currents connected to the electrojet. The midlatitude positive bay (MPB) index measures these effects. Both AL and MPB can be used to time auroral expansion onset. This work creates lists of MPB onsets and compares them with lists from other sources including onsets derived from IMAGE and Polar spacecraft and ground all‐sky camera data. The lists show that these measurements are nearly simultaneous with satellite expansion onsets. Substorms occur most frequently during the declining phase of the solar cycle, at the equinoxes, and after the passage of a stream interface in a corotating interaction region. Both MPB and SML indices reveal three substorm phases: growth, expansion, and recovery. In addition, the MPB onset list, the SML onset list, and the IMAGE auroral onset list reveal the presence of a 45‐min and a 2 3/4‐hr quasi‐periodicity in substorm occurrence. A comparison of the various lists reveals radical difference in their waiting time distributions and relatively low levels of association between events in the lists. The results suggest that none of the lists adequately capture the complexity of substorm activity and further refinements are needed in every algorithm used for onset detection.

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An optimum solar wind coupling function for the AL index

Cited by 59 publications

References 82 publications

Solar cycle dependence of substorm occurrence and duration: Implications for onset

Solar cycle dependence of substorm occurrence and duration: Implications for onset

Seasonal Variation of High‐Latitude Geomagnetic Activity in Individual Years

The Midlatitude Positive Bay Index and the Statistics of Substorm Occurrence

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