scholarly journals Spring-fall asymmetry of substorm strength, geomagnetic activity and solar wind: Implications for semiannual variation and solar hemispheric asymmetry

2011 ◽  
Vol 38 (6) ◽  
pp. n/a-n/a ◽  
Author(s):  
K. Mursula ◽  
E. Tanskanen ◽  
J. J. Love
Keyword(s):  
Solar Wind ◽  
Geomagnetic Activity ◽  
Hemispheric Asymmetry ◽  
Semiannual Variation

scholarly journals <I>a<sub>p</sub></I>-index solar wind driving function and its semiannual variations

2007 ◽  
Vol 25 (7) ◽  
pp. 1465-1469 ◽  
Author(s):  
A. A. Petrukovich ◽  
M. Y. Zakharov
Keyword(s):  
Solar Wind ◽  
Seasonal Variation ◽  
Response Function ◽  
Geomagnetic Activity ◽  
Linear Term ◽  
Response Functions ◽  
Driving Function ◽  
Semiannual Variation ◽  
Wind Input ◽  
Ap Index

Abstract. Semiannual variation of geomagnetic activity and ap-index in particular is supposed to consist of heliospheric factor (axial hypothesis and Russell-McPherron effect) and magnetospheric/ionospheric factor (equinoctical hypothesis). In our investigation we express ap-index as a magnetospheric response function to solar wind and IMF input. Seasonal variation in ap-index on average (1963–2003) is ~4 nT and consists of ~2.1–2.3 nT of magnetospheric/ionospheric part, 0.6–1.3 nT of heliospheric part (including 0.2–0.3 nT of R-M effect), 0.1–0.4 nT is due to the non-linear term. 90% confidence range of all estimates is ~0.1–0.25 nT. While autumn/spring magnetospheric response functions are almost identical, there is substantial difference between winter and summer functions. The increase of solar wind input in autumn and spring is also different by a factor of two.

scholarly journals Origins of the semiannual variation of geomagnetic activity in 1954 and 1996

2004 ◽  
Vol 22 (1) ◽  
pp. 93-100 ◽  
Author(s):  
E. W. Cliver ◽  
L. Svalgaard ◽  
A. G. Ling
Keyword(s):  
Solar Wind ◽  
Geomagnetic Activity ◽  
Solar Minimum ◽  
High Speed ◽  
Solar Wind Speed ◽  
Coronal Holes ◽  
Heliospheric Current Sheet ◽  
Solar Cycles ◽  
Axial Variation ◽  
Semiannual Variation

Abstract. We investigate the cause of the unusually strong semiannual variation of geomagnetic activity observed in the solar minimum years of 1954 and 1996. For 1996 we separate the contributions of the three classical modulation mechanisms (axial, equinoctial, and Russell-McPherron) to the six-month wave in the aam index and find that all three contribute about equally. This is in contrast to the longer run of geomagnetic activity (1868-1998) over which the equinoctial effect accounts for ∼70% of the semiannual variation. For both 1954 and 1996, we show that the Russell-McPherron effect was enhanced by the Rosenberg-Coleman effect (an axial polarity effect) which increased the amount of the negative (toward Sun) [positive (away from Sun)] polarity field observed during the first [second] half of the year; such fields yield a southward component in GSM coordinates. Because this favourable condition occurs only for alternate solar cycles, the marked semiannual variation in 1954 and 1996 is a manifestation of the 22-year cycle of geomagnetic activity. The 11-year evolution of the heliospheric current sheet (HCS) also contributes to the strong six-month wave during these years. At solar minimum, the streamer belt at the base of the HCS is located near the solar equator, permitting easier access to high speed streams from polar coronal holes when the Earth is at its highest heliographic latitudes in March and September. Such an axial variation in solar wind speed was observed for 1996 and is inferred for 1954. Key words. Magnetosphere (solar wind – magnetosphere interactions; storms and substorms)

Semiannual variation of the geomagnetic activity and solar wind parameters

1993 ◽  
Vol 20 (20) ◽  
pp. 2271-2274 ◽  
Author(s):  
M. Orlando ◽  
G. Moreno ◽  
M. Parisi ◽  
M. Storini
Keyword(s):  
Solar Wind ◽  
Geomagnetic Activity ◽  
Semiannual Variation ◽  
Solar Wind Parameters

ASSOCIATION BETWEEN VARIATIONS OF SOLAR WIND PARAMETERS AND GEOMAGNETIC ACTIVITY INDEX Ap IN 2003 - 2005

2011 ◽  
Vol 2 (3) ◽  
pp. 205-210 ◽  
Author(s):  
Igor Savel'evich Fal'kovich ◽  
M. R. Olyak ◽  
Nikolai Nikolaevich Kalinichenko ◽  
I. N. Bubnov
Keyword(s):  
Solar Wind ◽  
Geomagnetic Activity ◽  
Activity Index ◽  
Solar Wind Parameters ◽  
Geomagnetic Activity Index

scholarly journals Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity

2009 ◽  
Vol 27 (5) ◽  
pp. 1909-1914 ◽  
Author(s):  
A. Yoshida
Keyword(s):  
Solar Wind ◽  
Wind Velocity ◽  
Geomagnetic Activity ◽  
Physical Meaning ◽  
Solar Wind Velocity ◽  
Annual Variation ◽  
Geomagnetic Disturbance ◽  
Data Sets ◽  
Key Factor ◽  
Solar Wind Parameters

Abstract. Physical meaning of the equinoctial effect for semi-annual variation in geomagnetic activity is investigated based on the three-hourly am index and solar wind parameters. When the z component of the interplanetary magnetic field (IMF) in geocentric solar magnetospheric (GSM) coordinates is southward, am indices are well correlated with BsVx2, where Bs is the southward component of the IMF and Vx is the solar wind velocity in the sun-earth direction. The am-BsVx2 relationship, however, depends on the range of Vx2: the am in higher ranges of Vx2 tends to be larger than am in lower ranges of Vx2 for the same value of BsVx2 for both equinoctial and solstitial epochs. Using the data sets of the same Vx2 range, it is shown that distribution of points in the am-BsVx2 diagram at the solstitial epochs overlaps with that at the equinoctial epochs and the average am values in each BsVx2 bin in solstitial epochs are closely consistent with those in equinoctial epochs, if Vx2 for each point at solstices are reduced to Vx2sin2 (Ψ) where Ψ is the geomagnetic colatitude of the sub-solar point. Further, it is shown that monthly averages of the am index in the long period is well correlated with the values of sin2(ψ) for the middle day of each month. These findings indicate that the factor that contributes to the generation of geomagnetic disturbance is not the velocity of the solar wind, but the component of the solar wind velocity perpendicular to the dipole axis of the geomagnetic field. The magnitude of the perpendicular velocity component varies semi-annually even if the solar wind velocity remains constant, which is considered to be the long-missed key factor causing the equinoctial effect.

scholarly journals Introduction to special section on corotating solar wind streams and recurrent geomagnetic activity

2006 ◽  
Vol 111 (A7) ◽  
Author(s):  
Bruce T. Tsurutani ◽  
Robert L. McPherron ◽  
Walter D. Gonzalez ◽  
Gang Lu ◽  
Jose H. A. Sobral ◽  
...  
Keyword(s):  
Solar Wind ◽  
Geomagnetic Activity ◽  
Special Section
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