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1.
Plasma and magnetic field parameter variations across fast forward interplanetary shocks are analyzed during the last solar
cycle minimum (1995–1996, 15 shocks), and maximum year 2000 (50 shocks). It was observed that the solar wind velocity and
magnetic field strength variation across the shocks were the parameters better correlated with Dst. Superposed epoch analysis centered on the shock showed that, during solar minimum, B
z
profiles had a southward, long-duration variation superposed with fluctuations, whereas in solar maximum the B
z
profile presented 2 peaks. The first peak occurred 4 hr after the shock, and seems to be associated with the magnetic field
disturbed by the shock in the sheath region. The second peak occurred 19 hr after the shock, and seems to be associated with
the ejecta fields. The difference in shape and peak in solar maximum (Dst peak =−50 nT, moderate activity) and minimum (Dst peak =−30 nT, weak activity) in average Dst profiles after shocks are, probably, a consequence of the energy injection in the magnetosphere being driven by different
interplanetary southward magnetic structures. A statistical distribution of geomagnetic activity levels following interplanetary
shocks was also obtained. It was observed that during solar maximum, 36% of interplanetary shocks were followed by intense
(Dst≤−100 nT) and 28% by moderate (−50≤Dst <−100 nT) geomagnetic activity. During solar minimum, 13% and 33% of the shocks were followed by intense and moderate geomagnetic
activity, respectively. Thus, during solar maximum a higher relative number of interplanetary shocks might be followed by
intense geomagnetic activity than during solar minimum. One can extrapolate, for forecasting goals, that during a whole solar
cycle a shock has a probability of around 50–60% to be followed by intense/moderate geomagnetic activity. 相似文献
2.
We studied the cosmic ray intensity variation due to interplanetary magnetic clouds during an unusual class of low amplitude
anisotropic wave train events. The low amplitude anisotropic wave train events in cosmic ray intensity have been identified
using the data of ground based Deep River neutron monitor and studied during the period 1981–1994. Even though the occurrence
of low amplitude anisotropic wave trains does not depend on the onset of interplanetary magnetic clouds, but the possibility
of occurrence of these events cannot be overlooked during the periods of the interplanetary magnetic cloud events. It is observed
that the solar wind velocity remains higher (> 300) than normal and the interplanetary magnetic field B remains lower than normal on the onset of the interplanetary magnetic cloud during the passage of low amplitude wave trains.
It is also noted that the proton density remains significantly low during high solar wind velocity, which is expected. The
north south component of interplanetary magnetic field Bz turns southward to one day before the arrival of cloud and remains in the southward direction after the arrival of a cloud.
During these events the cosmic ray intensity is found to increase with increase of solar wind velocity. The superposed epoch
analysis of cosmic ray intensity for these events during the onset of interplanetary magnetic clouds reveals that the decrease
in cosmic ray intensity starts not at the onset of the cloud but after a few days. The cosmic ray intensity increases on arrival
of the magnetic cloud and decreases gradually after the passage of the magnetic cloud. 相似文献
3.
C. O. Lee J. G. Luhmann J. T. Hoeksema X. Sun C. N. Arge I. de Pater 《Solar physics》2011,269(2):367-388
The solar cycle 23 minimum period has been characterized by a weaker solar and interplanetary magnetic field. This provides
an ideal time to study how the strength of the photospheric field affects the interplanetary magnetic flux and, in particular,
how much the observed interplanetary fields of different cycle minima can be understood simply from differences in the areas
of the coronal holes, as opposed to differences in the surface fields within them. In this study, we invoke smaller source
surface radii in the potential-field source-surface (PFSS) model to construct a consistent picture of the observed coronal
holes and the near-Earth interplanetary field strength as well as polarity measurements for the cycles 23 and 22 minimum periods.
Although the source surface value of 2.5 R
⊙ is typically used in PFSS applications, earlier studies have shown that using smaller source surface heights generates results
that better match observations during low solar activity periods. We use photospheric field synoptic maps from Mount Wilson
Observatory (MWO) and find that the values of ≈ 1.9 R
⊙ and ≈ 1.8 R
⊙ for the cycles 22 and 23 minimum periods, respectively, produce the best results. The larger coronal holes obtained for the
smaller source surface radius of cycle 23 somewhat offsets the interplanetary consequences of the lower magnetic field at
their photospheric footpoints. For comparison, we also use observations from the Michelson Doppler Imager (MDI) and find that
the source surface radius of ≈ 1.5 R
⊙ produces better results for cycle 23, rather than ≈ 1.8 R
⊙ as suggested from MWO observations. Despite this difference, our results obtained from MWO and MDI observations show a qualitative
consistency regarding the origins of the interplanetary field and suggest that users of PFSS models may want to consider using
these smaller values for their source surface heights as long as the solar activity is low. 相似文献
4.
E. Eroshenko A. Belov H. Mavromichalaki G. Mariatos V. Oleneva C. Plainaki V. Yanke 《Solar physics》2004,224(1-2):345-358
During two extreme bursts of solar activity in March–April 2001 and October–November 2003, the ground-based neutron monitor
network recorded a series of outstanding events distinguished by their magnitude and unusual peculiarities. The important
changes that lead to increased activity initiated not with the sunspot appearance, but with the large-scale solar magnetic
field reconfiguration. A series of strong and moderate magnetic storms and powerful proton events (including ground-level
enhancements, GLE) were registered during these periods. The largest and most productive in the 23rd solar cycle, active region
486, generated a significant series of solar flares among which the 4 November 2003 flare (X28/3B) was the most powerful X-ray
solar event ever observed. The fastest arrival of the interplanetary disturbance from the Sun (after August 1972) and the
highest solar wind velocity and IMF intensity were recorded during these events. Within 1 week, three GLEs of solar cosmic
rays were registered by the neutron monitor network (28 and 29 October and 2 November 2003). In this work, we perform a tentative
analysis of a number of the effects seen in cosmic rays during these two periods, using the neutron monitor network and other
relevant data. 相似文献
5.
In the present work an analysis has been made of the extreme events occurring during July 2005. Specifically, a rather intense
Forbush decrease was observed at different neutron monitors all over the world during 16 July 2005. An effort has been made
to study the effect of this unusual event on cosmic ray intensity as well as various solar and interplanetary plasma parameters.
It is noteworthy that during 11 to 18 July 2005 the solar activity ranged from low to very active. Especially low levels occurred
on 11, 15, and 17 July whereas high levels took place on 14 and 16 July 2005. The Sun is observed to be active during 11 to
18 July 2005, the interplanetary magnetic field intensity lies within 15 nT, and solar wind velocity was limited to ∼500 kms-1.
The geomagnetic activity during this period remains very quiet, the Kp index did not exceed 5, the disturbance storm time
Dst index remains ∼-70 nT and no sudden storm commencement has been detected during this period. It is noted that for the
majority of the hours, the north/south component of the interplanetary magnetic field, Bz, remains negative, and the cosmic
ray intensity increases and shows good/high correlation with Bz, as the polarity of Bz tends to shift from negative to positive
values, the intensity decreases and shows good/high anti-correlation with Bz. The cosmic ray intensity tends to decrease with
increase of interplanetary magnetic field strength (B) and shows anti-correlation for the majority of the days.
Published in Astrofizika, Vol. 51, No. 2, pp. 255–265 (May 2008). 相似文献
6.
In the present work the cosmic ray data of three different neutron monitoring stations, Deep River, Inuvik, and Tokyo, located
at different geomagnetic cutoff rigidities and altitudes have been harmonically analyzed for the period 1980–95 for a comparative
study of diurnal semi-diurnal and tri-diurnal anisotropies in cosmic ray intensity in connection with the change in interplanetary
magnetic field Bz component and solar wind velocity on 60 quietest days. It is observed that the amplitudes of all the three
harmonics increase during the period 1982–84 at all the stations during the high speed solar wind stream epoch and remain
low during the declining phase of the stream. The amplitudes of the three harmonics have no obvious characteristics associated
with the time variation of magnitude of the Bz component. The phases of all the three harmonics have no time variation characteristics
associated with solar wind velocity and Bz.
Published in Astrofizika, Vol. 49, No. 4, pp. 651–664 (August 2006). 相似文献
7.
Measurement of the floor in the interplanetary magnetic field and estimation of the time-invariant open magnetic flux of the
Sun require knowledge of closed magnetic flux carried away by coronal mass ejections (CMEs). In contrast with previous papers,
we do not use global solar parameters to estimate such values: instead we identify different large-scale types of solar wind
for the 1976 – 2000 interval to obtain the fraction of interplanetary CMEs (ICMEs). By calculating the magnitude of the interplanetary
magnetic field B averaged over two Carrington rotations, the floor of the magnetic field can be estimated from the B value at a solar cycle minimum when the number of ICMEs is minimal. We find a value of 4.65±0.6 nT, in good agreement with
previous results. 相似文献
8.
Data of hourly interplanetary plasma (field magnitude, solar wind speed, and ion density), solar (sunspot number, solar radio
flux), and geomagnetic indices (Kp, Ap) over the period 1970-2010, have been used to examine the asymmetry between the solar
field north and south of the heliospheric current sheet (HCS). A persistent yearly north-south asymmetry of the field magnitude
is clear over the considered period, and there is no magnetic solar cycle dependence. There is a weak N-S asymmetry in the
averaged solar wind speed, exhibited well at times of maximum solar activities. The solar plasma is more dense north of the
current sheet than south of it during the second negative solar polarity epoch (qA < 0). Moreover, the N - S asymmetry in solar activity (Rz) can be statistically highly significant. The sign of the average N - S asymmetry depends upon the solar magnetic polarity.
The annual magnitudes of N - S asymmetry depend positively on the solar magnetic cycle. Most of the solar radio flux asymmetries
occurred during the period of positive IMF polarity. 相似文献
9.
Ronald L. Rosenberg 《Solar physics》1970,15(1):72-78
A simple model is used to present a unified picture of the polarity pattern of the interplanetary magnetic field observed during the solar cycle. Emphasis in this paper is on the field near solar maximum. The heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field is explained in terms of weak poloidal (dipolar) field sources in the sun's photosphere. Unlike the Babcock theory, the author hypothesizes that the dipolar field exists at equatorial latitudes (0–20°), too, (as well as in polar regions) and that the major source of the interplanetary magnetic field observed near the ecliptic plane is the dipolar field from equatorial latitudes. The polarity of the interplanetary field data taken in 1968 and in the first half of 1969 near solar maximum may possibly be explained in terms of a depression of the dipolar field boundary in space. The effect on the solar wind of the greater activity in the northern hemisphere of the sun that existed in 1968 and in the first half of 1969 is believed responsible for this hypothesized depression, especially near solar maximum, of the plane separating the + and - dipolar polarity below the solar equatorial plane in space. Predictions are made concerning the interplanetary field to be observed near the ecliptic plane in each portion of the next solar cycle. 相似文献
10.
Y.-M. Wang 《Solar physics》2004,224(1-2):21-35
The Sun’s large-scale external field is formed through the emergence of magnetic flux in active regions and its subsequent
dispersal over the solar surface by differential rotation, supergranular convection, and meridional flow. The observed evolution
of the polar fields and open flux (or interplanetary field) during recent solar cycles can be reproduced by assuming a supergranular
diffusion rate of 500 – 600 km2 s−1 and a poleward flow speed of 10 –20 m s−1. The nonaxisymmetric component of the large-scale field decays on the flow timescale of ∼1 yr and must be continually regenerated
by new sunspot activity. Stochastic fluctuations in the longitudinal distribution of active regions can produce large peaks
in the Sun’s equatorial dipole moment and in the interplanetary field strength during the declining phase of the cycle; by
the same token, they can lead to sudden weakenings of the large-scale field near sunspot maximum (Gnevyshev gaps). Flux transport
simulations over many solar cycles suggest that the meridional flow speed is correlated with cycle amplitude, with the flow
being slower during less active cycles. 相似文献
11.
During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the Magnetic Field Investigation (MFI) team
using Wind (1995 – 2003) solar wind plasma and magnetic field data from solar minimum through the maximum of cycle 23. The average occurrence
rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were
during the early part of solar cycle 23: (i) only four MCs were observed in 1999, and (ii) an unusually large number of MCs
(17 events) were observed in 1997, just after solar minimum. We also discuss the relationship between MCs, coronal mass ejections
(CMEs), and geomagnetic storms. During the period 1996 – 2003, almost 8000 CMEs were observed by SOHO-LASCO. The occurrence
frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number.
When we included “magnetic cloud-like structures” (MCLs, defined by Lepping, Wu, and Berdichevsky, 2005), we found that the
occurrence of the joint set (MCs + MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average
duration of the MCL structures is ~40% shorter than that of the MCs. The MCs are typically more geoeffective than the MCLs,
because the average southward field component is generally stronger and longer lasting in MCs than in MCLs. In addition, most
severe storms caused by MCs/MCLs with Dst
min≤ −100 nT occurred in the active solar period. 相似文献
12.
We study the temporal evolution of cosmic ray intensity during ~27-day Carrington rotation periods applying the method of superposed epoch analysis. We discuss about the average oscillations in the galactic cosmic ray intensity, as observed by ground based neutron monitors, during the course of Carrington rotation in low solar activity conditions and in different polarity states of the heliosphere (A<0 and A>0). During minimum and decreasing phases in low solar activity conditions, we compare the oscillation in one polarity state with that observed in other polarity state in similar phases of solar activity. We find difference in the evolution and amplitude of ~27-day variation during A<0 and A>0 epoch. We also compare the average variations in cosmic ray intensity with the simultaneous variations of solar wind parameters such as solar wind speed and interplanetary magnetic field strength. From the correlation analysis between the cosmic ray intensity and the solar wind speed during the course of Carrington rotation, we find that the correlation is stronger for A>0 than A<0. 相似文献
13.
Based on our analysis of the data fromthe global network of neutronmonitors for several events, we have found the times of
the first increases in count rate at individual stations that precede the main solar cosmic-ray enhancement. The onset time
of proton acceleration at the Sun has been determined from the appearance of a broad gamma-ray line with its maximum near
70 MeV that is generated during the decay of neutral pions, which, in turn, are produced when protons with energies above
300 MeV interact with the solar atmosphere. The time of the first recording of energetic protons at the Earth is delayed relative
to the time at which these protons appeared at the Sun by 60–300 s, i.e., by a value comparable to the difference between
the direct photon and particle propagation times. At least two conclusions follow from the existence of such “precursors”.
First, the protons begin to escape from the solar atmosphere into interplanetary space immediately after their acceleration.
Second, some of the protons traverse a path shorter than the nominal length of interplanetary magnetic field lines. 相似文献
14.
We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X- and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1 AU: standard solar wind or interplanetary coronal mass ejections (ICMEs). Our analysis shows that around 20 % of all particle events are detected when the spacecraft is immersed in an ICME. The correlation of the peak particle intensity with the projected speed of the SEP-associated coronal mass ejection is similar in the two IMF categories of proton and electron events, ≈?0.6. The SEP events within ICMEs show stronger correlation between the peak proton intensity and the soft X-ray flux of the associated solar flare, with correlation coefficient r=0.67±0.13, compared to the SEP events propagating in the standard solar wind, r=0.36±0.13. The difference is more pronounced for near-relativistic electrons. The main reason for the different correlation behavior seems to be the larger spread of the flare longitude in the SEP sample detected in the solar wind as compared to SEP events within ICMEs. We discuss to what extent observational bias, different physical processes (particle injection, transport, etc.), and the IMF configuration can influence the relationship between SEPs and coronal activity. 相似文献
15.
The minimum variance analysis of interplanetary coronal mass ejections (ICMEs) observed close to the Earth's orbit around
solar cycle 23 maximum (1998–2002) was performed. The ICMEs were classified in three categories: magnetic clouds (MC), undefined
ejecta (UE), and complex ejecta (CE). An analysis of the full ICMEs set shows that the average of minimum variance direction
inclination angle is 1.6°± 24.8° in relation to the ecliptic plane, with more than 33% of the events presenting inclination
angles lower than 10°. The average of minimum variance direction azimuthal angle (in relation to the Sun–Earth line) was 56°.
However, around 60% of the ICMEs presented an azimuthal angle lower than 30°, close to the radial direction. It was also observed
that the MC set had lower axial (intermediate variance) inclinations relative to the ecliptic plane than the UE and CE events.
The intermediate variance axis is close to 90° to the Sun–Earth line. The results obtained in the present analysis were also
compared with previous works, permitting a comparison of the ICMEs orientations in solar cycle 23 with previous sor cycles. 相似文献
16.
Forecasting space weather more accurately from solar observations requires an understanding of the variations in physical
properties of interplanetary (IP) shocks as solar activity changes. We examined the characteristics (occurrence rate, physical
parameters, and types of shock driver) of IP shocks. During the period of 1995 – 2001, a total of 249 forward IP shocks were
observed. In calculating the shock parameters, we used the solar wind data from Wind at the solar minimum period (1995 – 1997) and from ACE since 1998 including the solar maximum period (1999 – 2001). Most
of IP shocks (68%) are concentrated in the solar maximum period. The values of physical quantities of IP shocks, such as the
shock speed, the sonic Mach number, and the ratio of plasma density compression, are larger at solar maximum than at solar
minimum. However, the ratio of IMF compression is larger at solar minimum. The IP shock drivers are classified into four groups:
magnetic clouds (MCs), ejecta, high speed streams (HSSs), and unidentified drivers. The MC is the most dominant and strong
shock driver and 150 out of total 249 IP shocks are driven by MCs. The MC is a principal and very effective shock driver not
only at solar maximum but also at solar minimum, in contrast to results from previous studies, where the HSS is considered
as the dominant IP shock driver. 相似文献
17.
For 181 PCA's recorded during the years 1956–1969 the association with flares is studied. Both the number of events which
cannot be associated with any flare on the visible hemisphere, as well as the longitude distribution of identified proton
flares, lead to the conclusion that 25–30% of PCA's are caused by flares behind the western solar limb. PCA's of this kind
are mostly small. During the investigated years no PCA > 13 dB and possibly no PCA > 8.5 dB were caused by flares behind the
limb, while hardly 60% of PCA's < l dB had their origin on the visible hemisphere.
While the sources of GLE's and of PCA's in general, are centered around 50°W which corresponds to the average curvature of
the magnetic field lines in interplanetary space, the strongest PCA's (> 8.5 dB) show an anomalous longitude distribution
centered around ∼ 20°W. It is suggested that this anomaly may be a consequence of the fact that in strong PCA events the kinetic
energy density of protons below 100 MeV becomes comparable to the magnetic energy density in space, thus leading to a ‘straightening’
of the magnetic field lines. 相似文献
18.
The behavior of solar energetic particles (SEPs) in a shock – magnetic cloud interacting complex structure observed by the
Advanced Composition Explorer (ACE) spacecraft on 5 November 2001 is analyzed. A strong shock causing magnetic field strength and solar wind speed increases
of about 41 nT and 300 km s−1, respectively, propagated within a preceding magnetic cloud (MC). It is found that an extraordinary SEP enhancement appeared
at the high-energy (≥10 MeV) proton intensities and extended over and only over the entire period of the shock – MC structure
passing through the spacecraft. Such SEP behavior is much different from the usual picture that the SEPs are depressed in
MCs. The comparison of this event with other top SEP events of solar cycle 23 (2000 Bastille Day and 2003 Halloween events)
shows that such an enhancement resulted from the effects of the shock – MC complex structure leading to the highest ≥10 MeV
proton intensity of solar cycle 23. Our analysis suggests that the relatively isolated magnetic field configuration of MCs
combined with an embedded strong shock could significantly enhance the SEP intensity; SEPs are accelerated by the shock and
confined into the MC. Further, we find that the SEP enhancement at lower energies happened not only within the shock – MC
structure but also after it, probably owing to the presence of a following MC-like structure. This is consistent with the
picture that SEP fluxes could be enhanced in the magnetic topology between two MCs, which was proposed based on numerical
simulations by Kallenrode and Cliver (Proc. 27th ICRC
8, 3318, 2001b). 相似文献
19.
R. P. Kane 《Solar physics》2006,236(1):207-226
After increasing almost monotonically from sunspot minimum, sunspot activity near maximum falters and remains in a narrow
grove for several tens of months. During the 2–3 years of turmoil near sunspot maximum, sunspots depict several peaks (Gnevyshev
peaks). The spaces between successive peaks are termed as Gnevyshev Gaps (GG). An examination showed that the depths of the troughs varied considerably from one GG to the next in the same cycle, with magnitudes varying in a wide range (<1%
to ∼20%). In any cycle, the sunspot patterns were dissimilar to those of other solar parameters, qualitatively as well as
quantitatively, indicating a general turbulence, affecting different solar parameters differently. The solar polar magnetic
field reversal does not occur at the beginning of the general turmoil; it occurs much later. For cosmic ray (CR) modulation
which occurs deep in the heliosphere, one would have thought that the solar open magnetic field flux would play a crucial
role, but observations show that the sunspot GGs are not reflected well in the solar open magnetic flux, where sometimes only
one peak occurred (hence no GG at all), not matching with any sunspot peak and with different peaks in the northern and southern
hemispheres (north – south asymmetry). Gaps are seen in interplanetary parameters but these do not match exactly with sunspot
GGs. For CR data available only for five cycles (19 – 23), there are CR gaps in some cycles, but the CR gaps do not match
perfectly with gaps in the solar open magnetic field flux or in interplanetary parameters or with sunspot GGs. Durations are
different and/or there are variable delays, and magnitudes of the sunspot GGs and CR gaps are not proportional. Solar polar
magnetic field reversal intervals do not coincide with either sunspot GGs or CR gaps, and some CR gaps start before magnetic field reversals, which should not happen if the magnetic field reversals are the cause of the CR gaps. 相似文献
20.
The Grad–Shafranov reconstruction is a method of estimating the orientation (invariant axis) and cross section of magnetic
flux ropes using the data from a single spacecraft. It can be applied to various magnetic structures such as magnetic clouds
(MCs) and flux ropes embedded in the magnetopause and in the solar wind. We develop a number of improvements of this technique
and show some examples of the reconstruction procedure of interplanetary coronal mass ejections (ICMEs) observed at 1 AU by
the STEREO, Wind, and ACE spacecraft during the minimum following Solar Cycle 23. The analysis is conducted not only for ideal localized ICME
events but also for non-trivial cases of magnetic clouds in fast solar wind. The Grad–Shafranov reconstruction gives reasonable
results for the sample events, although it possesses certain limitations, which need to be taken into account during the interpretation
of the model results. 相似文献