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The terrestrial magnetosphere results from the interaction of the
terrestrial magnetic field and the magnetized solar wind plasma. The
magnetosphere shape, the design of the magnetic field lines, its current
systems and plasma concentration, all are controlled by the solar wind and
heliospheric disturbances. A favorable configuration of the heliospheric and
geomagnetic fields allows transfer of energy from solar wind to the terrestrial
magnetosphere resulting geomagnetic disturbances. A
geomagnetic storm is such a disturbance in the Earth's magnetosphere that
occurs when the interplanetary magnetic field turns southward and remains
southward for a prolonged period of time. The manifestations of storms are
strong deviations in the Earth's magnetic field from the quiet conditions that
extend over wide geographic areas: from high-latitude to mid-latitude and
equatorial regions.
The
Dst or disturbance storm time
index is a measure of geomagnetic activity used to assess the severity of
magnetic storms. It is expressed in nT and is based on the average value of the
horizontal component of the Earth's magnetic field measured hourly at four
near-equatorial geomagnetic observatories. The use of the Dst as an index of
storm strength is possible because the strength of the surface magnetic field
at low latitudes is inversely proportional to the energy content of the ring current, which increases during
geomagnetic storms.
Using Dst index, the geomagnetic storm
classification is:
•
-50 < Dst £ -30 ® minor (small)
storm (tipically substorm);
•
-100 < Dst £ -50 ® moderate
storm;
•
-150 < Dst £ -100 ®
intense (strong) storm;
•
- Dst £ -150 ® severe storm.
Three
distinct phases could be typically remarked during geomagnetic storms according
to the signatures in Dst:
·
Initial
phase lasting from minutes to hours -
Dst increases to positive values up to tens of nT;
·
Main
phase lasting to several hours -
Dst can reach negative values of hundreds of nT;
·
Recovery
phase lasting from tens of hours to a week -
Dst gradually returns to the normal level.
Initial
phase of some geomagnetic storms shows a sudden increase of Dst to positive
values up to tens of nT corresponding to the storm sudden commencement;
then it decreases sharply as the ring current intensifies. Other geomagnetic
storms begin by a gradual decrease in Dst – the gradual storm.
The geomagnetic storms are also
classified as recurrent and non-recurrent.The recurrent storms occur
every 27 days, corresponding to the Sun's rotation period. They are triggered
by the high-speed solar wind streams generated by coronal holes co-rotating
with the Sun combined with the southward oriented heliospheric magnetic field
(or, Bz negative).The non-recurrent geomagnetic storms are caused by
interplanetary disturbances driven by fast coronal mass ejections (CMEs) or
other eruptive solar events.
We
intended to set up a complex catalog of HSS and geomagnetic storms induced by
them for the whole 11-year solar cycle no. 23 (May 1996 – December 2008). All
geomagnetic storms having Dst minimum value less or equal to – 50 were
identified and associated to a corresponding HSS. The catalogue contains a file
for each year.
For
each event, let it be HSS or geomagnetic storm, its peculiar (main) parameters
are given (on rows). The catalog columns contain the following data:
-
Columns 1 – 4: HSS start by calendar data (Y – year, M – month, D – day, 3-H – 3-hour interval);
-
Column 5 – V0,
the minimum velocity (initial velocity) just before the stream, in km/s;
-
Column 6 – VMax,
maximum velocity of HSS, in km/s;
-
Column 7 – Dur, HSS duration, in days;
-
Column 8 – DVM, the velocity jump of HSS, DVM = VMax
– V0, in km/s;
-
Column 9 – IMF,
the dominant polarity of the interplanetary (heliospheric) magnetic field
during the stream;
-
Column 10 – the solar Source of HSS (CH – coronal hole; F – solar flare or another eruptive
solar phenomena);
-
Column 11 – Bz, the minimum (negative) value of Bz component
just before the maximum of the geomagnetic storm (minimum Dst value), in nT;
-
Column 12 – Dst_min,
the minimum Dst value during the geomagnetic storm, in nT;
-
Column 13 – t,
the moment of the Dst minimum, given by the calendar date: month, day and hour
(mm:dd:hh);
-
Column 14 – Type,
the storm type: SSC – storm with sudden commencement, GS – storm with gradual
commencement.
Generally (as a rule), one single geomagnetic storm
is induced by a HSS; in such cases the
HSS parameters and those of corresponding geomagnetic storm are written on the
same row. There are some cases when, during a complex HSS, two or even more
geomagnetic storms are registered; in these cases, the parameters of the
successive geomagnetic storms appear on successive rows having in columns 1-4
the date of the same HSS.
There are some geomagnetic storms with no any HSS
parameters in columns 1 – 10. They appeared following a jump of the solar wind
velocity <100 km/s (see criteria of HSS selection at: http://www.spacescience.ro/new1/HSS_Catalogue.html)
but a negative (southward) Bz that favored the particle injection from solar
wind to magnetosphere.
Table 1. Yearly distribution of the geomagnetic storms
|
Dst |
1996 |
1997 |
1998 |
1999 |
2000 |
2001 |
2002 |
2003 |
2004 |
2005 |
2006 |
2007 |
2008 |
Total |
|
-50>Dst>-100 |
10 |
18 |
20 |
22 |
28 |
18 |
24 |
44 |
15 |
20 |
6 |
5 |
3 |
236 |
|
-100>Dst>-150 |
1 |
5 |
7 |
3 |
6 |
7 |
12 |
4 |
7 |
8 |
2 |
0 |
0 |
62 |
|
Dst<-150 |
0 |
0 |
3 |
2 |
6 |
7 |
2 |
2 |
2 |
2 |
0 |
0 |
0 |
26 |
|
Total |
11 |
23 |
30 |
27 |
40 |
32 |
38 |
50 |
24 |
30 |
8 |
5 |
3 |
321 |
Statistics of the geomagnetic storms during solar cycle 23 is presented in Table 1. The columns of the table give the number of moderate, intense and severe storms for each year. The same statistics is given in figs. 1 and 2. The maximum geomagnetic activity was registered in 2003 (with the greatest dominance of the moderate storms). Secondary maxima were registered in: 1998 (a secondary maximum of the intense storm), 2000 (the maximum of the solar cycle 23, by sunspot relative number) and, 2005 (a secondary maximum for intense storms).
Figure 1. Yearly
distribution of the total number of geomagnetic storms during SC 23
Figure 2. The yearly distributions
of the geomagnetic storms by their intensity, during SC 23
Use of
these data in publications should be accompanied at minimum by mentioning the
source and authors of the catalog.
Acknowledgements. The authors acknowledge the OMNI data, obtained from
the GSFC/SPDF OMNI Web interface at http://omniweb.gsfc.nasa.gov
GS-HSS Catalogue
(1996-2008)
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008