R. Kerttula

Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events

Abstract We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.

Non-stationary Alfvén resonator: vertical profiles of wave characteristics

Abstract A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfven resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfven resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.

Storm‐time Pc1 activity at high and middle latitudes

We study structured and unstructured Pc1 pulsations observed at a high-latitude station (Sodankyla; L = 5.1) and a midlatitude station (Nurmijarvi; L = 3.3) during 18 storms occurring in low solar activity years (1976–1978 and 1984–1988). Pc1 activity was studied from the day of storm sudden commencement (denoted by day 0) onward during six consecutive days. While unstructured pulsations are only weakly affected, structured pulsations are greatly dependent on storm evolution. During the storm main phase they nearly vanish on the ground, despite strong wave activity in space. Structured Pc1 activity increases from day 0 to day 4 by a factor of about 4–5, reaching maximum occurrence on day 4 at both stations. Also, the average daily frequency of structured Pc1s increases from day 0 to a maximum on day 3 at Nurmijarvi or day 4 at Sodankyla. The diurnal distribution of structured Pc1s suffers a dramatic change during the storm. On days 1 and 2, structured pulsations are strongly concentrated in the evening sector, but during the later recovery (days 3–5) the activity shifts to the morning sector. The latitudinal similarity of structured Pc1 occurrence and the daily evolution of wave frequency argue against the model according to which the outward expansion of plasmapause causes the maximum wave occurrence on the ground on day 4. We also note that the strong maximum of structured Pels during the late storm recovery phase is not supported by the model calculations of the magnetospheric wave source or by direct observations of waves in space. Instead, we argue that the ionospheric resonator and propagation conditions which strongly affect wave observations on the ground are deteriorated during the storm main and early recovery phases, impeding wave propagation to the ground. The subsequent recovery of the ionospheric conditions leads to the maximum occurrence of structured Pc1s on the ground during the late storm recovery phase.

Effect of magnetic storm intensity on Pc1 activity at high and mid-latitudes

Abstract We study the properties of structured and unstructured Pc1 pulsations at a high-latitude station (Sodankyla; L =5.1) and a mid-latitude station (Nurmijarvi; L =3.3) during 18 storms occurring in low solar activity years. The storms were divided into two groups according to their intensity as measured by the minimum value of the D st index. Pc1 activity was studied from the day of the storm sudden commencement onwards during six consecutive days. Having recently published the average results for all 18 storms [Kerttula et al., J. Geophys. Res. (2000) in press], we concentrate here on the effect of magnetic storm intensity on wave properties. The source of structured Pc1s was found to be at lower latitudes during intense storms, in agreement with the lower latitude of the ring current during intense storms. Also, the source of unstructured Pc1s, the plasmasheet ions, was found to shift to lower latitudes during intense storms but this change was only observed in the early recovery phase. The great depletion of structured Pc1s on ground during the storm main and early recovery phase, which is in apparent disagreement with space observations and model calculations, is even more dramatic for intense storms. This further emphasizes the significance of the ionospheric conditions for wave observations on ground, and suggests that the depletion is due to deterioration of the ionospheric Alfven resonator during the storm main phase. Moreover, the results support the idea [Kerttula et al., J. Geophys. Res. 62 (2000) 299–309] that Pc1 occurrence maximum during late recovery phase is related to the improved ionospheric amplification and propagation conditions, rather than the outward expansion of the plasmapause.

New constraints on theories of Pc1 pearl formation

In this paper we study structured Pc1 pulsations (also called Pc1 pearls) observed on ground, concentrating on the relation between the pearl repetition period τ and the wave frequency f. Earlier studies suggest that the product τ f is roughly constant. We reexamine this relation and show that a simple inverse law is excluded. Instead, our observations suggest the relation τ ∝ f−p, with p = 0.59 ± 0.06, posing a new, strict constraint on theories of Pc1 pearl formation. We also study the L dependence of various combinations of τ and f using a model L value and extract additional constraints from these combinations. We discuss these constraints in the bouncing wave packet model of pearl formation and determine the range of allowed parameter values in this model. We present two models of energetic ions with different L distributions and show that one of them can be excluded by the constraints derived. We also discuss how to further improve on these constraints to better test the bouncing wave packet model and other theories of Pc1 pearl formation.

Effect of heavy ions on ponderomotive forces due to ion cyclotron waves

We study ponderomotive effects induced by the electromagnetic ion cyclotron (EMIC) waves in the Pel frequency band (0.2 to 5.0 Hz) in a two-ion (H+ and one heavy ion, e.g., He+) plasma. Near the dayside boundary of the magnetosphere, the ponderomotive forces lead to a noticeable accumulation of cold plasma along the field line in two regions of minimum magnetic field intensity located symmetrically around the equator. In the inner magnetosphere, one maximum of cold plasma at the equator is found. At frequencies less than the heavy ion gyrofrequency, the accumulation of cold plasma increases with increasing heavy ion concentration. At frequencies above the heavy ion gyrofrequency, the ponderomotive forces due to EMIC waves are enhanced because of a resonance at the stop band frequencies. We investigate the stop band structure of EMIC waves in a nondipolar magnetosphere and discuss the properties of wave propagation along the field line for different heavy ion plasma concentrations.

NUMERICAL SIMULATION OF THE HIGH-LATITUDE NON- STATIONARY IONOSPHERIC ALFVÉN RESONATOR DURING AN IPDP EVENT

The ionospheric Alfvén resonator (IAR) was numerically simulated under non-stationary ionospheric and magnetospheric conditions of the IPDP event of December 4, 1986. The full numerical wave method was applied using height profiles of the ionospheric plasma parameters obtained from the Scandinavian EISCAT radar measurements close to the Ivalo latitude. An attempt to model the inverse problem of numerical simulation—prolongation of the electron density profiles at altitudes above the ionospheric F layer—was made on the basis of the IAR simulation in correlation with the IPDP frequency increase. The change of the IAR wave characteristics during the substorm was illustrated by height profiles of the total wave amplitude and various polarization characteristics, taking into consideration the ordinary L-mode and the extraordinary R-mode waves for parallel and non-parallel incidence with respect to the magnetic field line.