- Email: [email protected]
Dynamics of long-period magnetic activity and energetic particle precipitation during the May 15, 1997 storm
Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
www.elsevier.com/locate/jastp
Dynamics of long-period magnetic activity and energetic particle precipitation during the May 15, 1997 storm V.A. Pilipenkoa; ∗ , O.V. Kozyrevaa , M.J. Engebretsonb , D.L. Detrickc , S.N. Samsonovd a Institute
of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia b Augsburg College, Minneapolis, MN 55454, USA c University of Maryland, College Park, MD 20742-2431, USA d Institute of Cosmophysical Research and Aeronomy, Yakutsk 677891, Russia
Received 4 October 2001; received in revised form 18 December 2001; accepted 18 January 2002
Abstract The large-scale behavior of quasi-steady magnetic activity and energetic particle precipitation and their variations in the nominal Pc5-6=Pi3 band (1.7–10 mHz) during the main phase of the intense magnetic storm on May 15, 1997, is studied using data from an array of magnetic and riometer stations in the Northern Hemisphere. The global azimuthal structure of magnetic and cosmic noise absorption disturbances at ∼64◦ geomagnetic latitude is revealed with the help of MLT-UT diagrams. The ionospheric westward electrojet intensi
1. Introduction One of the intriguing ideas currently and widely discussed is that the acceleration of relativistic electrons during strong magnetic storms might be related to ULF magnetic
Corresponding author. E-mail address: [email protected] (V.A. Pilipenko).
to a population of relativistic electrons in the inner magnetosphere. Speci
c 2002 Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 0 7 4 - 3
832
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
during the main phase of the May 15, 1997 magnetic storm: one was in the early morning hours, the other near dusk. The
dipoles, arranged in 7 × 7 matrix, which covers L values from 5.5 to 6.5 with time resolution 1 s. A 7 × 7 beam IRIS installed at Sondre Stromfjord (STF) gives spatial resolution of 20 km and time resolution of 10 s. Examination of meridional pro
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
833
Northern hemisphere
50
270
GAK DWS
50
CMO 70
SIM MCM
TIX
SMI
180
RAB ISL
GIL
70
70
360
STF NAQ
KIL SOD 50
KIR LRV
90
50
Fig. 1. Map of magnetic and riometer stations along 64◦ ± 2◦ geomagnetic latitude. Boxes denote magnetic stations and asterisks riometer stations. Codes of the stations used in the study are indicated.
Table 1 Selected magnetic and riometer stations Station
Fort McMurray Gillam Island lake Dawson Fort Smith Fort Simpson Rabbit lake S. Stromfjord Narsarsuaq Kiruna Sodankyla Tixie College HAARP Leirvogur Kilpisjarvi
Code
MCM GIL ISL DWS SMI SIM RAB STF NAQ KIR SOD TIK CMO GAK LRV KIL
Geographic
Geomagnetic
Midnight
Lat.
Long.
Lat.
Long.
56.66 56.85 53.88 64.05 58.00 61.75 58.22 67.02 61.18 67.84 67.42 71.59 64.91 62.41 64.18 69.05
248.79 265.58 265.32 220.89 246.00 238.77 256.32 309.28 314.57 20.42 26.39 128.78 212.14 214.88 338.30 20.79
64.6 67.2 64.3 66.0 65.4 67.5 67.7 73.2 66.3 64.6 63.8 65.7 65.1 63.1 65.1 65.8
307.4 331.8 331.9 271.9 303.2 292.2 317.1 41.7 43.9 103.1 107.7 196.9 263.4 267.3 67.8 104.3
08:03 06:35 06:35 10:33 08:20 09:03 07:27 02:16 02:07 19:46 20:04 16:04 11:14 00:55 00:08 21:15
834
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
4. Global MLT features of magnetic and CNA intensi!cations
(a)
(b)
Fig. 2. (a) Stacked magnetogram from 04 to 20 UT May 15, 1997 from stations along 64◦ latitude: TIX, SOD, LRV, NAQ, ISL, MCM, DWS, and CMO. (b) Stacked riogram from 04 to 20 UT May 15, 1997 from stations along 64◦ latitude: TIX, SOD, STF, ISL, MCM, DWS, and GAK.
To reveal the propagation features of magnetic and CNA intensi
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
835
Fig. 3. (a) Electrojet expansion displayed on MLT-UT plot from 04 to 20 UT May 15, 1997. Positive deviations of the H component are denoted by yellow, and negative by red and blue. (b) MLT-UT plot of magnetic ULF intensity in the 2–10 mHz band from 04 to 20 UT May 15, 1997.
836
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
Fig. 4. (a) MLT-UT plot of averaged CNA (low-pass
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
837
Fig. 4. (continued)
the drift direction of the morning-side energetic electrons. During the second disturbed interval (12–15 UT), the propagation pattern of CNA intensi
average value of the phase diAerence between KIR and SOD is ∼30◦ , which corresponds to an azimuthal wave number m = M’=M 7. It should be mentioned that the examination of Pi3 coherency at IMAGE stations with diAerent azimuthal separations (Pilipenko et al., 2001) showed that the amplitude scale of the disturbance is ¡ 10◦ , probably ∼5◦ . This scale does not
838
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
Fig. 5. Comparison of stacked high-pass
Fig. 6. Stacked H component magnetograms for the interval 06 –18 UT May 15, 1997 from azimuthally separated stations DWS, SIM, SMI, RAB, and GIL at 66◦ ± 1◦ latitude from the CANOPUS array.
6. Small-scale meridional CNA structure as observed by imaging riometers The small-scale meridional structure of particle precipitation is studied with the use of imaging riometers: HAARP at GAK and IRIS at KIL. A keogram (latitude-time dynamic plot) from the imaging riometer at KIL (Fig. 7a) indicates the time-latitude structure of electron precipitation during the substorm at 6 –10 UT: precipitation bursts start earlier at lower latitudes. This might be related to the earlier arrival at the observation of drifting electrons, injected during the substorm on the nightside, at smaller L shells. The keogram of CNA from multi-beam HAARP data (Fig. 7b) shows that during the second intensi
tor, the apparent propagation is diAerent from the substorm pattern, as observed by IRIS. It goes towards lower latitudes, that is, the precipitation bursts start earlier at higher latitudes. This may indicate that the increase of electron Fuxes in the inner magnetosphere during this magnetic intensi
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
839
Fig. 7. (a) Keogram of the absorption intensity from the IRIS at KIL for the interval 0630 –0930 UT. (b) Keogram of the absorption intensity from HAARP at GAK for the interval 1200 –1500 UT.
840
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
impulsive solar wind buAeting of the magnetosphere, or locally, by unstable Fuxes of energetic particles. During global excitation, only large-scale disturbances with small azimuthal wave numbers penetrate eAectively from an external source deep into the magnetosphere. Near the resonant conversion region, a natural
The assumed diAerence in the excitation mechanisms of storm-associated Pi3 variations and common Pc5 pulsations should be manifested in their azimuthal scales and propagation direction. Wave coherence between separated observation points is expected to be high for separations less than a typical wavelength. Our observational results indicate that Pi3 quasi-periodic magnetic disturbances during the storm main phase have a small transverse spatial scale (¡ 10◦ ) and propagate sunward, at least in the dusk sector, in the direction of energetic particle drift. The direct determination of the azimuthal Pi3 propagation in the dawn sector has not been possible because of the lack of a suNciently dense array of stations. Typical Pc5 waves observed during moderately disturbed periods propagate westward (anti-sunward) in the morning sector, and eastward in the dusk sector. They have a typical wave number m∼3 (Olson and Rostoker, 1978), and, correspondingly, much larger wavelength, that ◦ is ⊥ 120◦ . We note that there were observations of small-scale CNA pulsations in the Pc5 range which propagated eastward at a speed 0,3–2 km=s in the morning sector, which is opposite to the common phase velocity of dawn sector Pc5 pulsations (Kikuchi et al., 1988). This study also revealed that the auroral absorption drifted with the magnetospheric convection, i.e. westward in the afternoon sector and eastward in the morning sector. 7.2. The role of storm-time broad-band Pi3 in relativistic electron energization Energetic particle measurements on satellites during the May 15, 1997 magnetic storm showed that after the initial adiabatic dropout, the MeV-electron Fuxes increased and surpassed the pre-storm level in an hour. The analysis by Li et al. (1999) suggested that radial transport only cannot account for the observed increase of the outer zone electron radiation belt and another process, such as local heating, may be required to explain the enhancement of relativistic electrons. Baker et al. (1998) suggested that this local acceleration process, which accelerates electrons to multi-MeV energies on timescales of tens of minutes, may be related to strong ULF waves. The existing theories explaining acceleration of seed electrons up to relativistic energies by ULF oscillations invoke coherent drift-resonant acceleration of electrons by large-scale (e.g. m = 1–2) Pc5 pulsations (Elkington et al., 1999; Hudson et al., 2000) or magnetic pumping in the presence of random pitch angle scattering by axisymmetric (m = 0) MHD waves (Liu et al., 1999). We would like to stress the fact that the most intense storm-related Pi3 disturbances are physically diAerent from typical Pc5 pulsations. Thus, these models may interpret a gradual relativistic electron enhancement occurring during a storm recovery phase, when common Pc5 pulsations become pronounced, but not the fast increase during the main phase of a magnetic storm. Possibly other possible mechanisms of energy pumping from intense ULF turbulence to
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
electrons should be invoked. For example, Summers and Ma (2000) developed a quasi-linear theory accounting for the transit-time acceleration of electrons by fast mode MHD waves. Our examination of the amplitude/phase azimuthal structure has indicated small scales of the wave transverse structure, and, thus, that storm-related ULF disturbances cannot be modeled as compressional fast mode necessary for this mechanism to be operative. However, though the model of transit-time acceleration cannot be applied in a straightforward manner, we think that after a certain modi
841
electron density N at an altitude of the absorption layer as follows: −1 = 2 R Ne . This time constant was estimated to vary in the range 5 –30 s, depending on the characteristic energy of precipitating electrons, and the altitude of the absorption layer. Periodic oscillations of jz (t) ˙ exp(−i!t) produce periodic plasma density variations in the E-layer ionosphere with amplitude jz (t) n(t) = : (2) eMz(−i! + −1 ) (A). Let us assume that there are no magnetospheric waves, and ground magnetic signals are produced by the modulation of ionospheric current by periodic variations of precipitating electrons. We assume that a precipitation-disturbed region in the ionosphere is large as compared with the height h of the E-layer, so the geometrical attenuation factor can be neglected. The ionospheric electric
(3)
Taking into account that variations of the ionospheric conductance are caused by additional ionization by precipitating electrons, that is M#=# n=N; # %Mz, and the local conductivity in the E-layer % eN=B, we arrive at VE b(#) $0 (4) jz : (−i! + −1 ) Here VE = E=B is the convection velocity in the ionosphere produced by electric
(5) −1
In the low-frequency range ! , the comparison of these two mechanisms, described by Eqs. (4) and (5), gives that b(#) VE : l b(AW)
(6)
For typical average parameters, 10 s; 102 ; l 2 × 102 km, and VE 102 m=s, the ratio (6) is about unity, indicating that both mechanisms give comparable input in
842
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
ground magnetic signal. However, depending on actual parameters in diAerent situations the relationship (6) may vary noticeably. 8. Summary and conclusion During the main phase of a large magnetic storm we have observed two intensi
References Baker, D.N., Lepping, R.P., Blake, J.B., Callis, L.B., Rostoker, G., Singer, H.J., Reeves, G.D., 1998. A strong CME-related magnetic cloud interaction with the Earth’s magnetosphere: ISTP observations of rapid relativistic electron acceleration on May 15, 1997. Geophysical Research Letters 25, 2975.
Bar
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843 in electron precipitation and magnetic
843
Rosenberg, T.J., Weatherwax, A.T., Detrick, D.L., Lutz, L., 1999. High frequency active auroral research program imaging riometer diagnostic. Radio Science 34 (5), 1207–1215. Rostoker, G., Skopke, S., Baker, D.N., 1998. On the origin of relativistic electrons in the magnetosphere associated with some geomagnetic storms. Geophysical Research Letters 25, 3701– 3704. Saito, T., 1978. Long-period irregular magnetic pulsation, Pi3. Space Science Reviews 21, 427–467. Saka, O., Ijima, T., Yamagishi, H., Sato, N., Baker, D.N., 1992. Excitation of Pc5 pulsations in the morning sector by a local injection of particles in the magnetosphere. Journal of Geophysical Research 97, 10,693–10,701. Stasiewicz, K., Bellan, P., Chaston, C., Kletzing, C., Lysak, R., Maggs, J., Pokhotelov, O., Seyler, C., Shukla, P., StenFo, L., Streltsov, A., Wahlund, J.-E., 2000. Small scale alfvenic structure in the aurora. Space Science Reviews 92, 423–533. Summers, D., Ma, C.-Y., 2000. Rapid acceleration of electrons in the magnetosphere by fast-mode MHD waves. Journal of Geophysical Research 105, 15,887–15,895.
www.elsevier.com/locate/jastp
Dynamics of long-period magnetic activity and energetic particle precipitation during the May 15, 1997 storm V.A. Pilipenkoa; ∗ , O.V. Kozyrevaa , M.J. Engebretsonb , D.L. Detrickc , S.N. Samsonovd a Institute
of the Physics of the Earth, B. Gruzinskaya 10, Moscow 123995, Russia b Augsburg College, Minneapolis, MN 55454, USA c University of Maryland, College Park, MD 20742-2431, USA d Institute of Cosmophysical Research and Aeronomy, Yakutsk 677891, Russia
Received 4 October 2001; received in revised form 18 December 2001; accepted 18 January 2002
Abstract The large-scale behavior of quasi-steady magnetic activity and energetic particle precipitation and their variations in the nominal Pc5-6=Pi3 band (1.7–10 mHz) during the main phase of the intense magnetic storm on May 15, 1997, is studied using data from an array of magnetic and riometer stations in the Northern Hemisphere. The global azimuthal structure of magnetic and cosmic noise absorption disturbances at ∼64◦ geomagnetic latitude is revealed with the help of MLT-UT diagrams. The ionospheric westward electrojet intensi
1. Introduction One of the intriguing ideas currently and widely discussed is that the acceleration of relativistic electrons during strong magnetic storms might be related to ULF magnetic
Corresponding author. E-mail address: [email protected] (V.A. Pilipenko).
to a population of relativistic electrons in the inner magnetosphere. Speci
c 2002 Elsevier Science Ltd. All rights reserved. 1364-6826/02/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 2 ) 0 0 0 7 4 - 3
832
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
during the main phase of the May 15, 1997 magnetic storm: one was in the early morning hours, the other near dusk. The
dipoles, arranged in 7 × 7 matrix, which covers L values from 5.5 to 6.5 with time resolution 1 s. A 7 × 7 beam IRIS installed at Sondre Stromfjord (STF) gives spatial resolution of 20 km and time resolution of 10 s. Examination of meridional pro
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
833
Northern hemisphere
50
270
GAK DWS
50
CMO 70
SIM MCM
TIX
SMI
180
RAB ISL
GIL
70
70
360
STF NAQ
KIL SOD 50
KIR LRV
90
50
Fig. 1. Map of magnetic and riometer stations along 64◦ ± 2◦ geomagnetic latitude. Boxes denote magnetic stations and asterisks riometer stations. Codes of the stations used in the study are indicated.
Table 1 Selected magnetic and riometer stations Station
Fort McMurray Gillam Island lake Dawson Fort Smith Fort Simpson Rabbit lake S. Stromfjord Narsarsuaq Kiruna Sodankyla Tixie College HAARP Leirvogur Kilpisjarvi
Code
MCM GIL ISL DWS SMI SIM RAB STF NAQ KIR SOD TIK CMO GAK LRV KIL
Geographic
Geomagnetic
Midnight
Lat.
Long.
Lat.
Long.
56.66 56.85 53.88 64.05 58.00 61.75 58.22 67.02 61.18 67.84 67.42 71.59 64.91 62.41 64.18 69.05
248.79 265.58 265.32 220.89 246.00 238.77 256.32 309.28 314.57 20.42 26.39 128.78 212.14 214.88 338.30 20.79
64.6 67.2 64.3 66.0 65.4 67.5 67.7 73.2 66.3 64.6 63.8 65.7 65.1 63.1 65.1 65.8
307.4 331.8 331.9 271.9 303.2 292.2 317.1 41.7 43.9 103.1 107.7 196.9 263.4 267.3 67.8 104.3
08:03 06:35 06:35 10:33 08:20 09:03 07:27 02:16 02:07 19:46 20:04 16:04 11:14 00:55 00:08 21:15
834
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
4. Global MLT features of magnetic and CNA intensi!cations
(a)
(b)
Fig. 2. (a) Stacked magnetogram from 04 to 20 UT May 15, 1997 from stations along 64◦ latitude: TIX, SOD, LRV, NAQ, ISL, MCM, DWS, and CMO. (b) Stacked riogram from 04 to 20 UT May 15, 1997 from stations along 64◦ latitude: TIX, SOD, STF, ISL, MCM, DWS, and GAK.
To reveal the propagation features of magnetic and CNA intensi
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
835
Fig. 3. (a) Electrojet expansion displayed on MLT-UT plot from 04 to 20 UT May 15, 1997. Positive deviations of the H component are denoted by yellow, and negative by red and blue. (b) MLT-UT plot of magnetic ULF intensity in the 2–10 mHz band from 04 to 20 UT May 15, 1997.
836
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
Fig. 4. (a) MLT-UT plot of averaged CNA (low-pass
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
837
Fig. 4. (continued)
the drift direction of the morning-side energetic electrons. During the second disturbed interval (12–15 UT), the propagation pattern of CNA intensi
average value of the phase diAerence between KIR and SOD is ∼30◦ , which corresponds to an azimuthal wave number m = M’=M 7. It should be mentioned that the examination of Pi3 coherency at IMAGE stations with diAerent azimuthal separations (Pilipenko et al., 2001) showed that the amplitude scale of the disturbance is ¡ 10◦ , probably ∼5◦ . This scale does not
838
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
Fig. 5. Comparison of stacked high-pass
Fig. 6. Stacked H component magnetograms for the interval 06 –18 UT May 15, 1997 from azimuthally separated stations DWS, SIM, SMI, RAB, and GIL at 66◦ ± 1◦ latitude from the CANOPUS array.
6. Small-scale meridional CNA structure as observed by imaging riometers The small-scale meridional structure of particle precipitation is studied with the use of imaging riometers: HAARP at GAK and IRIS at KIL. A keogram (latitude-time dynamic plot) from the imaging riometer at KIL (Fig. 7a) indicates the time-latitude structure of electron precipitation during the substorm at 6 –10 UT: precipitation bursts start earlier at lower latitudes. This might be related to the earlier arrival at the observation of drifting electrons, injected during the substorm on the nightside, at smaller L shells. The keogram of CNA from multi-beam HAARP data (Fig. 7b) shows that during the second intensi
tor, the apparent propagation is diAerent from the substorm pattern, as observed by IRIS. It goes towards lower latitudes, that is, the precipitation bursts start earlier at higher latitudes. This may indicate that the increase of electron Fuxes in the inner magnetosphere during this magnetic intensi
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
839
Fig. 7. (a) Keogram of the absorption intensity from the IRIS at KIL for the interval 0630 –0930 UT. (b) Keogram of the absorption intensity from HAARP at GAK for the interval 1200 –1500 UT.
840
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
impulsive solar wind buAeting of the magnetosphere, or locally, by unstable Fuxes of energetic particles. During global excitation, only large-scale disturbances with small azimuthal wave numbers penetrate eAectively from an external source deep into the magnetosphere. Near the resonant conversion region, a natural
The assumed diAerence in the excitation mechanisms of storm-associated Pi3 variations and common Pc5 pulsations should be manifested in their azimuthal scales and propagation direction. Wave coherence between separated observation points is expected to be high for separations less than a typical wavelength. Our observational results indicate that Pi3 quasi-periodic magnetic disturbances during the storm main phase have a small transverse spatial scale (¡ 10◦ ) and propagate sunward, at least in the dusk sector, in the direction of energetic particle drift. The direct determination of the azimuthal Pi3 propagation in the dawn sector has not been possible because of the lack of a suNciently dense array of stations. Typical Pc5 waves observed during moderately disturbed periods propagate westward (anti-sunward) in the morning sector, and eastward in the dusk sector. They have a typical wave number m∼3 (Olson and Rostoker, 1978), and, correspondingly, much larger wavelength, that ◦ is ⊥ 120◦ . We note that there were observations of small-scale CNA pulsations in the Pc5 range which propagated eastward at a speed 0,3–2 km=s in the morning sector, which is opposite to the common phase velocity of dawn sector Pc5 pulsations (Kikuchi et al., 1988). This study also revealed that the auroral absorption drifted with the magnetospheric convection, i.e. westward in the afternoon sector and eastward in the morning sector. 7.2. The role of storm-time broad-band Pi3 in relativistic electron energization Energetic particle measurements on satellites during the May 15, 1997 magnetic storm showed that after the initial adiabatic dropout, the MeV-electron Fuxes increased and surpassed the pre-storm level in an hour. The analysis by Li et al. (1999) suggested that radial transport only cannot account for the observed increase of the outer zone electron radiation belt and another process, such as local heating, may be required to explain the enhancement of relativistic electrons. Baker et al. (1998) suggested that this local acceleration process, which accelerates electrons to multi-MeV energies on timescales of tens of minutes, may be related to strong ULF waves. The existing theories explaining acceleration of seed electrons up to relativistic energies by ULF oscillations invoke coherent drift-resonant acceleration of electrons by large-scale (e.g. m = 1–2) Pc5 pulsations (Elkington et al., 1999; Hudson et al., 2000) or magnetic pumping in the presence of random pitch angle scattering by axisymmetric (m = 0) MHD waves (Liu et al., 1999). We would like to stress the fact that the most intense storm-related Pi3 disturbances are physically diAerent from typical Pc5 pulsations. Thus, these models may interpret a gradual relativistic electron enhancement occurring during a storm recovery phase, when common Pc5 pulsations become pronounced, but not the fast increase during the main phase of a magnetic storm. Possibly other possible mechanisms of energy pumping from intense ULF turbulence to
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
electrons should be invoked. For example, Summers and Ma (2000) developed a quasi-linear theory accounting for the transit-time acceleration of electrons by fast mode MHD waves. Our examination of the amplitude/phase azimuthal structure has indicated small scales of the wave transverse structure, and, thus, that storm-related ULF disturbances cannot be modeled as compressional fast mode necessary for this mechanism to be operative. However, though the model of transit-time acceleration cannot be applied in a straightforward manner, we think that after a certain modi
841
electron density N at an altitude of the absorption layer as follows: −1 = 2 R Ne . This time constant was estimated to vary in the range 5 –30 s, depending on the characteristic energy of precipitating electrons, and the altitude of the absorption layer. Periodic oscillations of jz (t) ˙ exp(−i!t) produce periodic plasma density variations in the E-layer ionosphere with amplitude jz (t) n(t) = : (2) eMz(−i! + −1 ) (A). Let us assume that there are no magnetospheric waves, and ground magnetic signals are produced by the modulation of ionospheric current by periodic variations of precipitating electrons. We assume that a precipitation-disturbed region in the ionosphere is large as compared with the height h of the E-layer, so the geometrical attenuation factor can be neglected. The ionospheric electric
(3)
Taking into account that variations of the ionospheric conductance are caused by additional ionization by precipitating electrons, that is M#=# n=N; # %Mz, and the local conductivity in the E-layer % eN=B, we arrive at VE b(#) $0 (4) jz : (−i! + −1 ) Here VE = E=B is the convection velocity in the ionosphere produced by electric
(5) −1
In the low-frequency range ! , the comparison of these two mechanisms, described by Eqs. (4) and (5), gives that b(#) VE : l b(AW)
(6)
For typical average parameters, 10 s; 102 ; l 2 × 102 km, and VE 102 m=s, the ratio (6) is about unity, indicating that both mechanisms give comparable input in
842
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843
ground magnetic signal. However, depending on actual parameters in diAerent situations the relationship (6) may vary noticeably. 8. Summary and conclusion During the main phase of a large magnetic storm we have observed two intensi
References Baker, D.N., Lepping, R.P., Blake, J.B., Callis, L.B., Rostoker, G., Singer, H.J., Reeves, G.D., 1998. A strong CME-related magnetic cloud interaction with the Earth’s magnetosphere: ISTP observations of rapid relativistic electron acceleration on May 15, 1997. Geophysical Research Letters 25, 2975.
Bar
V.A. Pilipenko et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 831 – 843 in electron precipitation and magnetic
843
Rosenberg, T.J., Weatherwax, A.T., Detrick, D.L., Lutz, L., 1999. High frequency active auroral research program imaging riometer diagnostic. Radio Science 34 (5), 1207–1215. Rostoker, G., Skopke, S., Baker, D.N., 1998. On the origin of relativistic electrons in the magnetosphere associated with some geomagnetic storms. Geophysical Research Letters 25, 3701– 3704. Saito, T., 1978. Long-period irregular magnetic pulsation, Pi3. Space Science Reviews 21, 427–467. Saka, O., Ijima, T., Yamagishi, H., Sato, N., Baker, D.N., 1992. Excitation of Pc5 pulsations in the morning sector by a local injection of particles in the magnetosphere. Journal of Geophysical Research 97, 10,693–10,701. Stasiewicz, K., Bellan, P., Chaston, C., Kletzing, C., Lysak, R., Maggs, J., Pokhotelov, O., Seyler, C., Shukla, P., StenFo, L., Streltsov, A., Wahlund, J.-E., 2000. Small scale alfvenic structure in the aurora. Space Science Reviews 92, 423–533. Summers, D., Ma, C.-Y., 2000. Rapid acceleration of electrons in the magnetosphere by fast-mode MHD waves. Journal of Geophysical Research 105, 15,887–15,895.