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How the Intensity of Isolated Substorms Is Controlled by the Solar Wind Parameters

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How the Intensity of Isolated Substorms Is Controlled by the Solar Wind Parameters Vorobjev et al. Earth, Planets and Space (2018) 70:148 https://doi.org/10.1186/s40623-018-0922-5 FULL PAPER Open Access How the intensity of isolated substorms is controlled by the solar wind parameters Vyacheslav Georgievich Vorobjev1*, Elizaveta Evgenievna Antonova2,3 and Oksana Ivanovna Yagodkina1 Abstract Analysis of 163 isolated substorms showed that their intensity quantifed as a maximum absolute value of the AL index increases with an increase in the velocity and number density of the solar wind plasma and hence its dynamic pressure. Most of the coupling functions describing the energy loading to the magnetosphere, e.g., the Kan–Lee electric feld (EKL) and the Newell factor (dΦ/dt), do not include the dynamic pressure as an input parameter. Hav- ing examined the correlation between these functions and the dynamic pressure, we found that, surprisingly, while almost uncorrelated for any arbitrary time interval, both EKL and dΦ/dt correlate with the dynamic pressure within 1 h before the onset of isolated substorms. That is, an increase in the solar wind dynamic pressure is associated with an increase in the solar wind driving before the onset. We assume that the increase in the dynamic pressure as early as before substorm growth path creates the conditions inside the magnetosphere that impede the occurrence of sub- storms and increase the threshold for the instability leading to expansion onset, forcing the accumulation of greater amount of energy in the magnetosphere. This energy is released during substorm expansion, producing a more intense magnetic bay. Keywords: Isolated substorms, Substorm onset, The AL index, Solar wind plasma, Solar wind driving Introduction ring current development is unclear either. Te latter Te concept of classical substorm is based on the load- problem is especially important taking into account the ing of solar wind energy into the Earth’s magnetosphere recent results of Antonova et al. (2014, 2015, 2017) and followed by a sudden release of this energy during sub- Kirpichev et al. (2016), who showed that a signifcant storm expansion. To characterize the time interval of the part of the auroral oval maps to the plasma ring that sur- loading, which typically lasts 0.5–2 h, McPherron (1970) rounds the Earth. Accordingly, the transverse currents introduced the term “growth phase.” It was inferred from in this ring are a high-latitude continuation of the ring the number of observations that the growth phase starts current. when the Bz component of the interplanetary magnetic Te dependence of loading–unloading processes on feld (IMF) turns southward. Tis approach was subse- the parameters of the solar wind and IMF is one of the quently supported by many experimental and theoreti- unsolved problems of the magnetospheric substorm cal works (see, for example, Russell and McPherron 1973; physics. In particular, the relationship between intensity Shukhtina et al. 2005; Boakes et al. 2009). of an isolated substorm (characterized by the maximum However, despite extended eforts in the study of mag- absolute value of the AL index) and these parameters is netospheric substorms, there are many key problems, unknown. Results of previous studies are ambiguous which remain unsolved, such as a location of substorm and often contradictory. For example, Kallio et al. (2000) onset and the mechanism leading to a sudden release of concluded that substorm intensity is determined by the energy. A relationship between substorm processes and direct input of energy during the expansive phase, which contradicts the results of Lyons (1996) and Lyons et al. (1997), who found that a number of substorm onsets are *Correspondence: [email protected] associated with a northward turning of the IMF Bz com- 1 Polar Geophysical Institute, Apatity, Murmansk Region, Russia Full list of author information is available at the end of the article ponent. Shukhtina et al. (2005) and Milan et al. (2009) © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Vorobjev et al. Earth, Planets and Space (2018) 70:148 Page 2 of 7 showed that the intensity of substorms is determined solar wind V, N and P are responsible for the amount of by the total magnetic fux transported from the dayside energy uploaded during the growth phase and subse- magnetosphere to the tail before substorm onset. How- quently released during the expansive phase, and hence ever, later Li et al. (2013) demonstrated that the inten- for the intensity of a substorm. Terefore, we study the sity of substorms correlates well with the electric feld behavior of the IMF components, as well as of the solar determined by the Kan–Lee formula (Kan and Lee 1979), wind velocity, number density, and dynamic pressure, which is independent of the total amount of energy preceding isolated substorms of diferent intensities, in uploaded to the magnetosphere during the growth phase. order to examine their infuence on the energy loading to Vorobjev et al. (2016) made a statistical study of the load- the magnetosphere before substorm onset. Te depend- ing–unloading processes of the magnetosphere during ence of the loading on the SYM/H index of geomagnetic isolated substorms and showed that there is a strong rela- activity is also investigated. tionship between the integral values of energy uploaded to the magnetosphere during the growth and expansive Data analysis phases and the substorm intensity quantifed as a total In the current study, we used the list of 163 isolated sub- energy of auroral precipitation. storms of diferent intensities available at http://pgia.ru/ Terefore, to establish the factor controlling substorm lang/en/data. Te isolated substorms included in this list intensity, which varies in a wide range, is a fundamental were selected using diurnal variations and 1-min values unsolved problem. It is clear that the solar wind plasma of the AL index during the winter seasons from 1995 to and the IMF are the main sources of the magnetospheric 2013, according to the following criteria: substorm energy. Conventionally, it is assumed that the Bz component of the IMF is responsible for substorm 1. Te time interval between two consecutive sub- generation. Te solar wind velocity (V) and number storms should be at least 3 h. density (N), and hence the dynamic pressure (P), do not 2. Te AL magnitude should not exceed 1500 nT. change signifcantly on the timescales of substorm and 3. Te substorm duration should not be less than 3 h. usually are not considered as independent sources of 4. Te time of the end of substorm corresponds to the substorms. However, there are some studies, which point instant at which the absolute value of AL is < 0.2 of to a possible importance of the solar wind kinetic energy the absolute value of AL minimum. for substorm development. For example, Barkhatov et al. (2017) showed that the prediction of the AL index varia- Te selection of substorms in accordance with cri- tions using the neural networks is more successful when teria 1–3 was performed by visual inspection of the AL the input parameters contain not only the IMF compo- index diurnal variations, while the time of onset (T0) nents, but also the dynamic pressure integrated over corresponding to the nth AL index was determined 2–3 h before substorm onset. At the same time, Newell as follows. For substorms with minimum AL less et al. (2013, 2016) studied the long-term efects of sub- than − 300 nT, T0 corresponded to the time for which storms, including diurnal, seasonal, and annual vari- |(AL AL )| > 60 nT and |(AL AL )| > 100 nT. n − n−1 n+1 − n−1 ations, and showed that the probability to predict the For weak isolated substorms (ALmin > − 300 nT) a simpler appearance of substorm series increases when the solar criterion was used: |(AL AL )| > 30 nT. (Please see n − n−1 wind velocity is used as a unique input parameter instead Vorobjev et al. (2016) for details.) of more complicated functions, which include the com- Te solar wind parameters, IMF, and geomagnetic indi- ponents of the IMF. Te efect of the dynamic pressure ces were taken from 1-min OMNI database provided by is not signifcant. Te results by Maggiolo et al. (2017) the NASA’s Space Physics Data Facility. For each sub- suggest that the solar wind velocity’s dominant impact storm an 8-h interval (4 h before and 4 h after T0) was on geomagnetic activity is caused by the compression inspected to eliminate the events for which data gaps regions at the interface of fast/slow solar wind regimes, of more than 10-min duration were present in the time which are very geo-efective, as they are associated with series. high solar wind pressure and strong interplanetary mag- netic feld. Generation conditions for substorms of diferent In the present study, we test the hypothesis that prior intensities to the substorm growth phase the solar wind plasma All selected substorms were divided into three parameters (generally N and Pd) not only determine the groups, depending on the minimum value of magnetospheric state, but also to a large extent deter- AL variation: weak (ALmin > 300 nT), moder- mine the intensity of a following isolated substorm. Te − ate ( 600 nT < ALmin < 300 nT), and strong characteristics of this magnetospheric state defned by − − (− 1500 nT < ALmin < − 600 nT). For each group we Vorobjev et al.
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