- Email: [email protected]
On the large-scale effects of two interplanetary shocks on the associated particle events
On the large-scale effects of two interplanetary shocks on the associated particle events A. M. HERAS,*$ B. SANAHuJA,?‘$ Z. K. SMITH.: T. DETMAN$ and M. DRYERI * Space Science Department of ESA. ESTEC. Postbus 399. 2200 AC Noordwijk zh, The Netherlands : t Department d’Astronomia i Meteorologia. Universitat de Barcelona. Av. Diagonal 647,08028-Barcelona. Spain ; f: NOAA Space Environment Laboratory, 315 Broadway, Boulder. CO 80303. U.S.A. ; 9Laboratori d’Astrofisica de I’Institut d’Estudis Catalans, Barcelona, Spain
(Received in Jinul form 16 April 199I ) Abstract-An evolutionary model, including particle and shock propagation through the interplanetary medium. has been used to reproduce the evolution of the flux and anisotropy in the upstream region of two low-energy particle events observed by ISEE-3. These events. on 24 April 1979 and I8 February 1979, originated at solar (helio)longitudes r 50 apart. By fitting the observed particle fluxes and anisotropies, the conditions for the propagation of the particles through the interplanetary medium and the injection rates at the shock have been determined as a function of time. The results are discussed in terms of the interplanetary magnetic field connection between the observer and the shock front and they are related to the heliolongitude of the parent solar activity.
INTRODUCTION Energetic Storm Particle (ESP) events are often seen accompanying interplanetary shocks. It is usually assumed that the evolution of these events basically depends on the plasma parameters where the shock is detected. Several authors (SANAHUJA and DOMINGO, 1987; CANE et al.. 1988; DOMINGO et al., 1989) have pointed out that the heliolongitude of the Parent Solar Activity (PSA: flare or filament disappearance) can also play an important role in the particle flux and anisotropy evolution of these events. This influence is essentially dependent on both the interplanetary magnetic field (IMF) connection between the observer and the PSA site, and the relative orientation of the shock with respect to the observer. Up to now, the study of these phenomena has not included quantitative models, mainly because of the complexity in modelling simultaneously the evolution of the shock and particle acceleration and propagation. The numerical model presented in HERAS et al. (1991) is a first approach to the simulation of a particle event taking into account the large-scale topology of the shock and the IMF. We apply this model here to reproduce the evolution of the flux and the anisotropy profiles of an ESP event of the ‘East’ type (the observer is not connected with the oncoming shock. through the IMF, until shortly before the detection of the shock ; see SANAHUJAand DOMINGO, 1987). The importance of the PSA heliolongitude in the temporal profiles of these particle events is clearly illustrated when comparing these results with the ones derived
in HERAS et al. (1991) for a ‘West’ event (the IMF connection is established at the onset of the event or well in advance of the observation of the shock; SANAHUJAand DOMINGO. 1987; CANE et ~1.. 1988).
THE 18 FEBRUARY1979PARTICLE EVENT An interplanetary shock was observed by ISEEon 18 February 1979 at 0219UT, associated with an increase in the particle flux in the energy range 351600 keV. The solar origin of this event has been identified as a 3B flare located at 16’N. 59’ E (CANE, 1985), that reached the maximum in H, at 0152 UT on 16 February. We have reproduced the observed particle flux and anisotropy temporal profiles by using the numerical model described in HERAS ef al. (1991). This model includes the simulation of the shock propagation, particle injection both at the solar corona and at the shock front and particle propagation through the interplanetary medium. Simulation of the interplanetary
shock
To simulate the shock, we used the 24D MHD timedependent code which simulates disturbances that propagate out through the interplanetary medium from the inner boundary of 18 solar radii to beyond 1 AU. The equations and the numerical scheme used in the code are described by WV et al. (1983). A description of the method of computation, input pulse models and steady state used are given in SMITH and DRYER (1990).
1033
1034
A. M. HERAS et al.
t= 22 hours
1979
FEBRUARY
18
Fig. 1. Results of the simulation for the shock observed at BEE-3 on 18 February 1979. The density contours (log,, cm-‘) are represented at 22 h and t, = 37 h. The locations of ISEE-3, Helios-I and Helios2 are also shown.
Figure 1 shows the locations of Helios-1, Helios-2 and ISEE- spacecraft relative to the PSA (indicated by an arrow), as well as the density contours obtained from the simulation 22 and 37 h after the occurrence of the flare. As can be seen, these spacecraft were well positioned to observe the interplanetary shock (disturbance). Indeed, a shock was observed by both Helios-2 and ISEE-3. At Helios-1, however, in the time interval during which the shock would have been expected, a substantial velocity jump was observed, which is not a well identified shock and which was preceded by an interesting ‘void’, or ‘hole’. That is, the density decreased abruptly by about an order of magnitude, while the velocity and temperature increased slightly and became unsettled for the duration of the density dropout. This signature could perhaps be explained as the resultant of the interaction of a shock with a rarefaction in the solar wind. We
thus used this feature to establish the approximate timing and upstream velocity. On the basis of the parametric study by SMITH and I~RYER (1990), we were able to select, within a few iterations, the input pulse parameters at the near-Sun lower boundary (18 R,) which produce an interplanetary shock similar to that observed by the spacecraft. These input shock parameters are: the initial shock velocity, v, = 1500 km SK’, longitudinal width, w = 90”, and temporal duration, z = 1.5 h. Table 1 gives the comparison of the data with the simulation for the shock transit times (TT) and maximum solar wind velocities downstream (sunward side) of the shock (V). The transit time is measured between the computational inner boundary of 18 solar radii and the spacecraft, and the subscripts Hl, H2, IC3 refer to Helios-1, Helios-2 and ISEE-3, respectively.
1035
Effects of two interplanetary shocks Table 1. Comparison between the observations and the values derived from the simulation of the propagation of the 18 February 1979 shock.
v "1 Simulation Data
830 900
vF?2 v,CX (km s-‘) 700 650
490 500
TTH,
nH?
nXZ,
(hours) .____30.0 35.0 47.0 29.0 38.5 48.5
Simulation of the particle event
Figure 2 shows the particle flux and anisotropy observed by ISEE- in five energy channels (solid curve). The IMF was pointing towards the Sun during the event; thus, negative anisotropy means that particles are mostly flowing outwards from the Sun (SANDERSON et al., 1985a). There is no enhancement in the flux after the occurrence of the flare, but an important increase both in the flux and anisotropy is observed only about six hours before the passage of the shock. Since solar particles are not detected, this event can be considered a pure ‘ESP’ event, that is, observed particles are only accelerated at the interplanetary shock. Therefore, to perform the simulation we have only taken into account injection at the shock front and neglected injection at the solar corona. As can be seen in Fig. 2, the particle flux increase starts suddenly and at the same time in all the energy channels. If we observed the first particles coming from the shock just after the magnetic connection with the spacecraft was established, a different delay for each energy should be expected. Therefore, what we actu-
620-1000keV
18
384-620keV
17
ally observe is a sudden connection between the shock and the spacecraft due to a change in the IMF direction, the flux tube being already popuIated with shock accelerated particles. That means that it is not possible to know accurately from the observations when the first particles accelerated at the shock start propagating along this field line. Since in our model the propagation of the particles is calculated considering only one flux tube, we simulate the evolution of the flux and anisotropy as if the direction of the IMF were steady during all the event. For the same reason, we have not taken into account the small increase in the flux starting at x 1200 UT on 17 February. It is likely that this is due to particle injection in a tube of flux different from the one considered in our model. The free parameters of our model are q, A,, (both for particle propagation) and Q (injection at the shock front), as described in HERAS efal. (1991). The value of q = 1.8 has been taken from the power spectrum slopes of the observed magnetic field, while A,,and Q have been obtained from the fit to the observations.
RESULTS AND DISCUSSION
The simulation of the shock and IMF provides the time r, when the magnetic connection between ISEE3 and the shock front is estabIished. For this event. t, = 37& I h (t = 0 is set at the time of the flare occurrence), when we also assume that particle injection starts at the shock front. The second panel of Fig. 1 shows the position of the shock with respect to the
238-384keV
18
17
FEBRUARY
147-238keV
f8
17
18
17
I8
17
19’79
Fig. 2. Particle flux and anisotropy profiles detected by ISEE- at five energy channels (solid curve). The fit obtained from the model is represented by a dashed curve, the time of magnetic connection by a solid vertical line and the time of the passage of the shock by a dashed vertical line.
1036
A. M. HEI
"EAST"
Table 2. Mean free path at each energy for the 18 February 1979 event. Energy
,,,3
i,, (AU
"WEST-CENTRAL MERlDliiid" 620-IOOOkeV ’ “1 1 I
ISEEI ““‘I
c
a) 1979 FEB.18
62%1000 keV 384620 keV 238-384 keV 147-238 keV
1.3 1.2 1.2
-L
>
i
VY
E
x at this time. Figure 2 displays the particle flux and anisotropy fit obtained from the model for the five selected energy channels (dashed curve), t, and the time of the shock passage being indicated by a solid and dashed vertical line, respectively. As has been pointed out above, BEE-3 only observes part of the history of the flux tube determined by the IMF line considered in our calculations. That is the reason why the simulation’s flux and anisotropy increases start sooner than the observed ones in the three higher energy channels. The beginning of the particle flux and anisotropy enhancements is in agreement with the observations for all the channels except for the one corresponding to the lowest energy. Therefore, the actual t, might be smaller than the time obtained from the shock simulation. Table 2 displays the values of i,, derived from the fit. The high observed anisotropies lead to very high values of the mean free path, which decreases with energy. The values of Q are represented in Fig. 3 as a function of energy for each interval of time. The injection rate of particles at the shock increases as the energy decreases, and it is more important as the shock approaches ISEE-3. spacecraft
'\ ', +
*‘,
\I
1o-37.
't,
102
103
E(keV) Fig. 3. Particle injection rate at the shock as a function energy,where(+)37 47 h.
of
;I_ \I I’ -y’,,!, , ,I,by
to-‘-
2
,0-361
is: J
,o” -
.ti N
Y a
b) 1979APR.24
22 IO’-
1.0 0.8
91-147keV
lo*
3
lo-2-
2
2.00
I
I
ix
1.00
‘0
0.00
$:
!J
E
I(
m
z,
-1.00 -2.001 ' 1 0
20
1 ' ’ 0 1I 40
60
20
40
60
TIME (HOURS) Fig. 4. Flux and anisotropy temporal profiles for (a) the 18 February 1979 event and (b) the 24 April 1979 event. The origin of times is set at the time of the respective PSA. The passage of the shock is indicated by a vertical line.
Comparison with the 24 April 1979 event The simulation of the 24 April 1979 event is described in HERASet al. (1991). Its PSA was located at 10, E in contrast with the location of the 18 February PSA, 59”E. The difference between both PSA heliolongitudes with respect to the observer implies a different evolution of the flux and anisotropy profiles, as is shown in Fig. 4. Panels (a) and (b) represent the particle flux and anisotropy for the 18 February and 24 April events, respectively, where t = 0 is set at the time of the respective PSA. The former flux profile is typical of an ‘East event’ and the second one is a prototype of a ‘West-Central Meridian event’. Note that although the anisotropy is negative in one event and positive in the other, both particle fluxes point outwards from the Sun. Solar accelerated particles are only observed in the 24 April event, shortly after the PSA occurrence, and the high long-lasting anisotropy, indicating particle injection at the shock front. starts at z 20 h for the 24 April event and at ~40 h at the 18 February event. The anisotropy keeps very high during this latter ESP event but decreases slowly until the arrival of the shock during the 24 April event.
Effects of two interplanetary 1.20
I
_t 0.80L1I >
l
I
I
I
I
1
I
1
.
+
---1919 FEB.18
,+g&h
-
4;+&+ 0.40-
*?@+ +4?+
0
I
1919 APR.24
o.oo0.00
*y yc**Ic WUW
10.00 20.00 30.00 40.00
50.00
TIME (HOURS) Fig. 5. Bottom and middle panel : spatial coordinates of the point of intersection between the shock and the IMF line that oasses through ISEE- for the 18 Februarv 1979 event (dashed line) and-the 24 April 1979 event (solid line), as a function of time. Top panel : Evolution of VR at the point of magnetic connection for the 18 February 1979 event (*) and the 24 April 1979 event (+) (see text).
In Fig.
5, the position
of the point
of intersection
the IMF line that passes through the spacecraft is shown as a function of time for the 24 April (solid line) and for the 18 February (dashed line) events. The heliocentric distance of this point is represented in the bottom panel and the angle between the vector ‘centre of the Sun-intersecting point’ and the CM in the middle panel. The value of the solar wind velocity ratio VR = (V,(d) - V,(u))/ V,(u) at the point of intersection is shown in the top panel (U and d stand for upstream and downstream, respectively). Provided that we consider that the observed particles are accelerated at this point, the fact that the magnetic connection with the shock is established z 20 h later for the 18 February event between
the shock
front
and
shocks
than for the 24 April event is particularly important to understand the differences between the respective profiles commented above. Moreover, the value of VR increases for both events as the shock approaches ISEE-3, but it remains much lower for the 18 February event than for the 24 April event. This shock evolves at the point of intersection from quasi-perpendicular to quasi-parallel conditions as it propagates outwards from the Sun. Nevertheless. for the 18 February event, the angle between the normal to the surface of the shock and the IMF remains between 76 and 90” as given by the shock simulation (the observational value at ISEE- is 66”; BAVASSANO-CATTANEO etal., 1986); thus, it is always quasi-perpendicular. Considering the calculated mean free paths, an outstanding difference is that, for the 18 February event. it is not necessary to assume a region of enhanced scattering upstream of the shock as in the last part of the 24 April event. Moreover, the high mean free paths during the 18 April event rule out the existence of significant turbulence upstream of the shock. This fact is in agreement with the observations of waves upstream of interplanetary shocks (SANDERSON etu/., 1985b; TSURUTANI etal., 1983) which are more commonly associated with quasi-parallel than with quasiperpendicular shocks. The particle injection rate at the shock is about two orders of magnitude higher, and the corresponding spectrum is harder during the 24 April event than for the 18 February event. The injection rate increases as the shock propagates outwards during both events, which could be related to the evolution of VR at the magnetic connection point. All of these results confirm the idea that large-scale topological evolution of interplanetary shocks, which depend a great deal on the position of the PSA with respect to the observer, plays an important role in the associated particle events. AcknuMlrulyenzents~We are grateful to T. R. Sanderson. Space Science Department of ESA. for provision of the particle flux anisotropy data and R. Schwenn for making HeliosI and -2 plasma data available. We thank J. Beeck from the University of Bremen, and R. D. Zwickl from NOAA for their assistance and useful discussions, A.M.H. was holding a fellowship from the Ministerio de Eduacion y Ciencia (Spain) during the first part ofthis work and an ESA research fellowship during the second.
REFERENCES BAVASSANO-CATTANEO M. B., TSURUTANIB. T., SMITHE.J. and LIN R. P. CANE H.V. CANEH. V., REAMESD. V. and VON ROSENVINGE T. T. DOMINGO V., SANAHUJA B. and HERAS A. M.
1037
1986
J. geophys. Res. 91, 11929.
1985
J. geophys. Res. 90, 19 1. J. geophys. Res. 93, 9555. .ddz,. Space Res. 9(4). 191.
1988 1989
1038
A. M. HERAS et al.
HERASA. M., SANAHLUA B., SMITHZ. K., DETMANT. and DRYER M. SANAHUJA B. and DOMINGOV. SANDER~~N T. R., REINHARDR., VAN NES P. and WENZELK.-P. SANDERSON T. R., REINHARDR., VAN NES P., WENZELK.-P., SMITHE. J. and TSURUTANI B. T. SMITHZ. K. and DRYERM. TSURUTANI B. T., SMITHE. J. and JONJLS D. E. Wu S. T., DRYERM. and HAN S. M.
1991
J. aimos. terr. Phys. 53, 10
1987 1985a
J. geophys. Res. 92,128O. J. geophys. Res. 90, 19.
1985b
J. geophys. Res. 90,3973.
1990 1983 1983
Solar Phys. 129, 387. J. geophys. Res. 88, 5645. Solar Phys. 84, 395.
(Received in Jinul form 16 April 199I ) Abstract-An evolutionary model, including particle and shock propagation through the interplanetary medium. has been used to reproduce the evolution of the flux and anisotropy in the upstream region of two low-energy particle events observed by ISEE-3. These events. on 24 April 1979 and I8 February 1979, originated at solar (helio)longitudes r 50 apart. By fitting the observed particle fluxes and anisotropies, the conditions for the propagation of the particles through the interplanetary medium and the injection rates at the shock have been determined as a function of time. The results are discussed in terms of the interplanetary magnetic field connection between the observer and the shock front and they are related to the heliolongitude of the parent solar activity.
INTRODUCTION Energetic Storm Particle (ESP) events are often seen accompanying interplanetary shocks. It is usually assumed that the evolution of these events basically depends on the plasma parameters where the shock is detected. Several authors (SANAHUJA and DOMINGO, 1987; CANE et al.. 1988; DOMINGO et al., 1989) have pointed out that the heliolongitude of the Parent Solar Activity (PSA: flare or filament disappearance) can also play an important role in the particle flux and anisotropy evolution of these events. This influence is essentially dependent on both the interplanetary magnetic field (IMF) connection between the observer and the PSA site, and the relative orientation of the shock with respect to the observer. Up to now, the study of these phenomena has not included quantitative models, mainly because of the complexity in modelling simultaneously the evolution of the shock and particle acceleration and propagation. The numerical model presented in HERAS et al. (1991) is a first approach to the simulation of a particle event taking into account the large-scale topology of the shock and the IMF. We apply this model here to reproduce the evolution of the flux and the anisotropy profiles of an ESP event of the ‘East’ type (the observer is not connected with the oncoming shock. through the IMF, until shortly before the detection of the shock ; see SANAHUJAand DOMINGO, 1987). The importance of the PSA heliolongitude in the temporal profiles of these particle events is clearly illustrated when comparing these results with the ones derived
in HERAS et al. (1991) for a ‘West’ event (the IMF connection is established at the onset of the event or well in advance of the observation of the shock; SANAHUJAand DOMINGO. 1987; CANE et ~1.. 1988).
THE 18 FEBRUARY1979PARTICLE EVENT An interplanetary shock was observed by ISEEon 18 February 1979 at 0219UT, associated with an increase in the particle flux in the energy range 351600 keV. The solar origin of this event has been identified as a 3B flare located at 16’N. 59’ E (CANE, 1985), that reached the maximum in H, at 0152 UT on 16 February. We have reproduced the observed particle flux and anisotropy temporal profiles by using the numerical model described in HERAS ef al. (1991). This model includes the simulation of the shock propagation, particle injection both at the solar corona and at the shock front and particle propagation through the interplanetary medium. Simulation of the interplanetary
shock
To simulate the shock, we used the 24D MHD timedependent code which simulates disturbances that propagate out through the interplanetary medium from the inner boundary of 18 solar radii to beyond 1 AU. The equations and the numerical scheme used in the code are described by WV et al. (1983). A description of the method of computation, input pulse models and steady state used are given in SMITH and DRYER (1990).
1033
1034
A. M. HERAS et al.
t= 22 hours
1979
FEBRUARY
18
Fig. 1. Results of the simulation for the shock observed at BEE-3 on 18 February 1979. The density contours (log,, cm-‘) are represented at 22 h and t, = 37 h. The locations of ISEE-3, Helios-I and Helios2 are also shown.
Figure 1 shows the locations of Helios-1, Helios-2 and ISEE- spacecraft relative to the PSA (indicated by an arrow), as well as the density contours obtained from the simulation 22 and 37 h after the occurrence of the flare. As can be seen, these spacecraft were well positioned to observe the interplanetary shock (disturbance). Indeed, a shock was observed by both Helios-2 and ISEE-3. At Helios-1, however, in the time interval during which the shock would have been expected, a substantial velocity jump was observed, which is not a well identified shock and which was preceded by an interesting ‘void’, or ‘hole’. That is, the density decreased abruptly by about an order of magnitude, while the velocity and temperature increased slightly and became unsettled for the duration of the density dropout. This signature could perhaps be explained as the resultant of the interaction of a shock with a rarefaction in the solar wind. We
thus used this feature to establish the approximate timing and upstream velocity. On the basis of the parametric study by SMITH and I~RYER (1990), we were able to select, within a few iterations, the input pulse parameters at the near-Sun lower boundary (18 R,) which produce an interplanetary shock similar to that observed by the spacecraft. These input shock parameters are: the initial shock velocity, v, = 1500 km SK’, longitudinal width, w = 90”, and temporal duration, z = 1.5 h. Table 1 gives the comparison of the data with the simulation for the shock transit times (TT) and maximum solar wind velocities downstream (sunward side) of the shock (V). The transit time is measured between the computational inner boundary of 18 solar radii and the spacecraft, and the subscripts Hl, H2, IC3 refer to Helios-1, Helios-2 and ISEE-3, respectively.
1035
Effects of two interplanetary shocks Table 1. Comparison between the observations and the values derived from the simulation of the propagation of the 18 February 1979 shock.
v "1 Simulation Data
830 900
vF?2 v,CX (km s-‘) 700 650
490 500
TTH,
nH?
nXZ,
(hours) .____30.0 35.0 47.0 29.0 38.5 48.5
Simulation of the particle event
Figure 2 shows the particle flux and anisotropy observed by ISEE- in five energy channels (solid curve). The IMF was pointing towards the Sun during the event; thus, negative anisotropy means that particles are mostly flowing outwards from the Sun (SANDERSON et al., 1985a). There is no enhancement in the flux after the occurrence of the flare, but an important increase both in the flux and anisotropy is observed only about six hours before the passage of the shock. Since solar particles are not detected, this event can be considered a pure ‘ESP’ event, that is, observed particles are only accelerated at the interplanetary shock. Therefore, to perform the simulation we have only taken into account injection at the shock front and neglected injection at the solar corona. As can be seen in Fig. 2, the particle flux increase starts suddenly and at the same time in all the energy channels. If we observed the first particles coming from the shock just after the magnetic connection with the spacecraft was established, a different delay for each energy should be expected. Therefore, what we actu-
620-1000keV
18
384-620keV
17
ally observe is a sudden connection between the shock and the spacecraft due to a change in the IMF direction, the flux tube being already popuIated with shock accelerated particles. That means that it is not possible to know accurately from the observations when the first particles accelerated at the shock start propagating along this field line. Since in our model the propagation of the particles is calculated considering only one flux tube, we simulate the evolution of the flux and anisotropy as if the direction of the IMF were steady during all the event. For the same reason, we have not taken into account the small increase in the flux starting at x 1200 UT on 17 February. It is likely that this is due to particle injection in a tube of flux different from the one considered in our model. The free parameters of our model are q, A,, (both for particle propagation) and Q (injection at the shock front), as described in HERAS efal. (1991). The value of q = 1.8 has been taken from the power spectrum slopes of the observed magnetic field, while A,,and Q have been obtained from the fit to the observations.
RESULTS AND DISCUSSION
The simulation of the shock and IMF provides the time r, when the magnetic connection between ISEE3 and the shock front is estabIished. For this event. t, = 37& I h (t = 0 is set at the time of the flare occurrence), when we also assume that particle injection starts at the shock front. The second panel of Fig. 1 shows the position of the shock with respect to the
238-384keV
18
17
FEBRUARY
147-238keV
f8
17
18
17
I8
17
19’79
Fig. 2. Particle flux and anisotropy profiles detected by ISEE- at five energy channels (solid curve). The fit obtained from the model is represented by a dashed curve, the time of magnetic connection by a solid vertical line and the time of the passage of the shock by a dashed vertical line.
1036
A. M. HEI
"EAST"
Table 2. Mean free path at each energy for the 18 February 1979 event. Energy
,,,3
i,, (AU
"WEST-CENTRAL MERlDliiid" 620-IOOOkeV ’ “1 1 I
ISEEI ““‘I
c
a) 1979 FEB.18
62%1000 keV 384620 keV 238-384 keV 147-238 keV
1.3 1.2 1.2
-L
>
i
VY
E
x at this time. Figure 2 displays the particle flux and anisotropy fit obtained from the model for the five selected energy channels (dashed curve), t, and the time of the shock passage being indicated by a solid and dashed vertical line, respectively. As has been pointed out above, BEE-3 only observes part of the history of the flux tube determined by the IMF line considered in our calculations. That is the reason why the simulation’s flux and anisotropy increases start sooner than the observed ones in the three higher energy channels. The beginning of the particle flux and anisotropy enhancements is in agreement with the observations for all the channels except for the one corresponding to the lowest energy. Therefore, the actual t, might be smaller than the time obtained from the shock simulation. Table 2 displays the values of i,, derived from the fit. The high observed anisotropies lead to very high values of the mean free path, which decreases with energy. The values of Q are represented in Fig. 3 as a function of energy for each interval of time. The injection rate of particles at the shock increases as the energy decreases, and it is more important as the shock approaches ISEE-3. spacecraft
'\ ', +
*‘,
\I
1o-37.
't,
102
103
E(keV) Fig. 3. Particle injection rate at the shock as a function energy,where(+)37
of
;I_ \I I’ -y’,,!, , ,I,by
to-‘-
2
,0-361
is: J
,o” -
.ti N
Y a
b) 1979APR.24
22 IO’-
1.0 0.8
91-147keV
lo*
3
lo-2-
2
2.00
I
I
ix
1.00
‘0
0.00
$:
!J
E
I(
m
z,
-1.00 -2.001 ' 1 0
20
1 ' ’ 0 1I 40
60
20
40
60
TIME (HOURS) Fig. 4. Flux and anisotropy temporal profiles for (a) the 18 February 1979 event and (b) the 24 April 1979 event. The origin of times is set at the time of the respective PSA. The passage of the shock is indicated by a vertical line.
Comparison with the 24 April 1979 event The simulation of the 24 April 1979 event is described in HERASet al. (1991). Its PSA was located at 10, E in contrast with the location of the 18 February PSA, 59”E. The difference between both PSA heliolongitudes with respect to the observer implies a different evolution of the flux and anisotropy profiles, as is shown in Fig. 4. Panels (a) and (b) represent the particle flux and anisotropy for the 18 February and 24 April events, respectively, where t = 0 is set at the time of the respective PSA. The former flux profile is typical of an ‘East event’ and the second one is a prototype of a ‘West-Central Meridian event’. Note that although the anisotropy is negative in one event and positive in the other, both particle fluxes point outwards from the Sun. Solar accelerated particles are only observed in the 24 April event, shortly after the PSA occurrence, and the high long-lasting anisotropy, indicating particle injection at the shock front. starts at z 20 h for the 24 April event and at ~40 h at the 18 February event. The anisotropy keeps very high during this latter ESP event but decreases slowly until the arrival of the shock during the 24 April event.
Effects of two interplanetary 1.20
I
_t 0.80L1I >
l
I
I
I
I
1
I
1
.
+
---1919 FEB.18
,+g&h
-
4;+&+ 0.40-
*?@+ +4?+
0
I
1919 APR.24
o.oo0.00
*y yc**Ic WUW
10.00 20.00 30.00 40.00
50.00
TIME (HOURS) Fig. 5. Bottom and middle panel : spatial coordinates of the point of intersection between the shock and the IMF line that oasses through ISEE- for the 18 Februarv 1979 event (dashed line) and-the 24 April 1979 event (solid line), as a function of time. Top panel : Evolution of VR at the point of magnetic connection for the 18 February 1979 event (*) and the 24 April 1979 event (+) (see text).
In Fig.
5, the position
of the point
of intersection
the IMF line that passes through the spacecraft is shown as a function of time for the 24 April (solid line) and for the 18 February (dashed line) events. The heliocentric distance of this point is represented in the bottom panel and the angle between the vector ‘centre of the Sun-intersecting point’ and the CM in the middle panel. The value of the solar wind velocity ratio VR = (V,(d) - V,(u))/ V,(u) at the point of intersection is shown in the top panel (U and d stand for upstream and downstream, respectively). Provided that we consider that the observed particles are accelerated at this point, the fact that the magnetic connection with the shock is established z 20 h later for the 18 February event between
the shock
front
and
shocks
than for the 24 April event is particularly important to understand the differences between the respective profiles commented above. Moreover, the value of VR increases for both events as the shock approaches ISEE-3, but it remains much lower for the 18 February event than for the 24 April event. This shock evolves at the point of intersection from quasi-perpendicular to quasi-parallel conditions as it propagates outwards from the Sun. Nevertheless. for the 18 February event, the angle between the normal to the surface of the shock and the IMF remains between 76 and 90” as given by the shock simulation (the observational value at ISEE- is 66”; BAVASSANO-CATTANEO etal., 1986); thus, it is always quasi-perpendicular. Considering the calculated mean free paths, an outstanding difference is that, for the 18 February event. it is not necessary to assume a region of enhanced scattering upstream of the shock as in the last part of the 24 April event. Moreover, the high mean free paths during the 18 April event rule out the existence of significant turbulence upstream of the shock. This fact is in agreement with the observations of waves upstream of interplanetary shocks (SANDERSON etu/., 1985b; TSURUTANI etal., 1983) which are more commonly associated with quasi-parallel than with quasiperpendicular shocks. The particle injection rate at the shock is about two orders of magnitude higher, and the corresponding spectrum is harder during the 24 April event than for the 18 February event. The injection rate increases as the shock propagates outwards during both events, which could be related to the evolution of VR at the magnetic connection point. All of these results confirm the idea that large-scale topological evolution of interplanetary shocks, which depend a great deal on the position of the PSA with respect to the observer, plays an important role in the associated particle events. AcknuMlrulyenzents~We are grateful to T. R. Sanderson. Space Science Department of ESA. for provision of the particle flux anisotropy data and R. Schwenn for making HeliosI and -2 plasma data available. We thank J. Beeck from the University of Bremen, and R. D. Zwickl from NOAA for their assistance and useful discussions, A.M.H. was holding a fellowship from the Ministerio de Eduacion y Ciencia (Spain) during the first part ofthis work and an ESA research fellowship during the second.
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