Nightside Magnetic Impulsive Events: Statistics and Possible Mechanisms

1. The 5th International conference «Trigger effects in geosystems» Nightside Magnetic Impulsive Events:Statistics and Possible Mechanisms A.V. Vorobev1,2, V.A. Pilipenko2,3 and M.J. Engebertson4 1Ufa State Aviation Technical University, Ufa, Russia 2Geophysical Center of the Russian Academy of Sciences, Moscow, Russia 3Institute of Physics of the Earth, Moscow, Russia 4Augsburg University, Minneapolis, USA 4 – 7 June, Moscow 2019 This work was supported by grant 16-17-00121 from the Russian Science Foundation. Magnetometer data from MACCS are available at http://space.augsburg.edu/maccs, from AUTUMNX at http://autumn.athabascau.ca, and from CARISMA at http://www.carisma.ca

2. MAGNETIC IMPULSE EVENTS MIEs (Magnetic Impulse Events ) are pulses of large amplitude (of the order of tens to hundreds of nT in both horizontal and vertical components) with a charac-teristic duration of 5 to 10 minutes and an amplitude of several hundred nT, localized in space with scales of about first hundred kilometers. Fig. 1. Map of Eastern Arctic Canada showing the locations of the 8 ground magnetometers used in this study. Fig. 2. Example of nightside MIEs recorded at station IGL on Oct. 10, 2015

3. DISTRIBUTION LAWSANALYSIS According to calculation and evaluation of the Kolmogorov's criterion [Bolshev L.N. and Smirnov N.V., 1983] the hypothesis, that values distribution fits to lognormal law, can be rejected with a level of significance not exceeding 0.01%.

4. HISTOGRAMS AND PROBABILITY DISTRIBUTION FUNCTIONS a b c d e f

5. ANALYSIS OF THE PROBABILITY DISTRIBUTION FUNCTIONS • The probabilities of impulse with dB/dt>10 nT/s are at IGL 12 %; SALU 11.0 % and FCHU 6.5 %. • Knowing the average number of events during a year one obtains that for dB/dt>10 nT/s we should observed at IGL: 12 events per year, at SALU 42 events per year and at FCHU 27 events per year. • The probabilities of extreme impulse with dB/dt>50 nT/s are IGL 0.007 %; SALU 0.005 % and FCHU 0.0006 %. Therefore, the expected annual rate of extreme dB/dt is IGL: 0.7 events per year; SALU: 1.92 events per year and FCHU 0.25 events per year.

6. HISTOGRAMS AND PROBABILITY DISTRIBUTION FUNCTIONS a b Fig. 2: a – Histograms of the probability distribution PDF(B) (grey bars) and cumulative distribution (exceedance function) P(>B) (solid black lines) of data from IGL. The best fit log-normal approximations are denoted by solid red lines for B and dashed red lines for P(>B); b – The same format, but for dB/dt. Blue dashed lines show the power-law approximation of the distribution tails.

7. ANALYSIS OF THE PROBABILITY DISTRIBUTION FUNCTIONS • The probabilities of impulses with dB/dt>10 nT/s are 38% at IGL. • Knowing the average number at IGL of events (201) during two (2015, 2017) years one obtains that 38 events with dB/dt>10 nT/s should occur per year. The probability of extreme impulse with dB/dt>50 nT/s are 0.001%. • Therefore, the expected annual rate of such extreme dB/dt is 0.1 events per year. The estimated probability of very high dB/dt gives us a possibility to evaluate a probability of extreme GIC in a given region.

8. ANALYSIS OF THE PROBABILITY DISTRIBUTION FUNCTIONS Generalized normal distribution where PDF(x) is probability density function;  is a shape parameter: at  = 2 distribution fits to normal low, at  = 1 – to Laplace distribution law (thus distribution tails are heavier than normal when  < 2 and lighter than normal when  > 2). Fig. 3. The statistical distributions PDF(t) and P(>t) for the time delay t between the substorm intensification and MIE occurrence ( = 1.83).

9. CONCLUSION The statistical analyses presented may be of some help in efforts to determine the mechanisms responsible for the nighttime MIEs. Comparison of the statistical characteristics of different time series enables one to speculate about the similarity of their physical nature. The comparison of the statistical distributions of impulse amplitudes of both |B| and |dB/dt| shows that PDFs at all stations fit best the log-normal distribution. The fact that amplitudes of nightside MIEs in the range of two magnitudes are described by the same law indicates that these impulsive disturbances are not accidental, but they are manifestation of some organized physical process. For example, the fact that the PDF appears to be log-normal may indicate that this distribution is formed as a result of a multiplicative stochastic effect. According to many observations, turbulence in the Earth magnetotail often has a log-normal form. Such a coincidence may indicate that the turbulence of the near-Earth plasma is largely responsible for the variability of the geomagnetic field on the time scale of these events.

10. CONCLUSION Regularities of distributions of amplitudes of dayside MIEs have been investigated in [Klain B.I., N.A. Kurazhkovskaya, 2006] using observations from a number of high-latitude observatories. Tails of functions of impulse amplitude statistical distributions were reasonably well approximated by a power function. The majority of statistical distributions of dayside MIEs’ amplitudes had the exponent a>2 that is typical of chaotic regimes called strong intermittent turbulence. Comparison of the amplitude statistics for dayside MIEs and SCs showed that statistics for both types of impulses were well approximated by power-law function, whereas the correlation coefficient between the approximation curve and experimental data was 0.95–0.99 for MIEs and 0.90–0.97 for SSCs. However, the power-law approximation was applied in [Kurazhkovskaya N.A., Klain B.I., 2016] to a rather narrow range of variables: the ratio between maximal and minimal values was ~3 only. Indeed, the power-law approximation applied to tails of PDF of nighttime MIEs visually coincides with the log-normal approximation. In a wider range of variables (maximum to minimum ratio is ~17) the power-law approximation is insufficient, and PDF is better modeled by the log-normal distribution.

11. CONCLUSION The mechanism of formation of a quasi-periodic series of MIEs, observed as Ps6 pulsations, has not been understood yet. Their periodicity 10-20 min is larger than typical Alfven eigenfrequency of field lines at auroral latitudes (3-10 min). Ps6 magnetic pulsations are often associated with the omega bands forms which typically occur at the poleward edge of the diffuse aurora. The substorm activations are typically associated with an activation of the most poleward arc system (called Poleward Boundary Intensifications – PBI) followed by the North-South aligned protrusions of auroral forms. A quasi-periodic sequence of these auroral forms is associated with Ps6 magnetic pulsations. Highly likely the PBI auroral events are the ionospheric manifestation of Bursty Bulk Flows (BBFs) in the magnetotail. Therefore, nighttime MIEs during substorms can occur in association with magnetotail BBF, whereas multiple BBF could be a driver of Ps6 pulsations.