T. Pikkarainen

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.

Role of the plasmapause and ionosphere in the generation and propagation of pearl pulsations

The source of Pc 1 (pearl) pulsations observed in the course of the local morning hours on 7 December 1977 has been determined by the amplitude and group delay methods. The frequency of pulsations exhibit the typical diurnal variation with the maximum frequency during dawn hours. The source location of pearls during every 1-h interval is compared with the position of the plasmapause inferred from the GEOS I measurements and from previous statistical analysis. It is shown that the source of high-frequency pulsations (f > 1 Hz) is well inside the plasmapause whereas low-frequency pulsations (f < 1 Hz) occur near the plasmapause. The source of pulsations is displaced to higher L-values in the course of the local morning hours and this displacement is associated with the decrease of the frequency of pulsations. The source displacement is much more pronounced than the simultaneous movement of the plasmapause position. These observations imply that the model of the Pc1 generation which locates the source only at the plasmapause has serious shortcomings. A model is discussed which takes into account the generation of Pc1 pulsations also well inside the plasmapause and the properties of the waveguide propagation of waves in the ionspheric duct.

Pc 1 induced electromagnetic lift of background plasma in the magnetosphere

We discuss the ponderormotive redistribution of ions along geomagnetic field lines due to the action of Pc 1 pulsations, using a simple diffusion equilibrium model. The field-aligned component of the ponderomotive force is derived for Alfven waves as well as for ion cyclotron waves propagating in a multicomponent plasma. Our theory gives a possibility for a better understanding of the problem and a convenient way to make rough estimates of the ponderomotive efficiency of Pc 1 waves under concrete magnetospheric conditions. Qualitative analysis of equations and satellite information on the intensity of wave fields show that the ponderomotive forces can have a pronounced effect on plasma. Ground-based observations of Pc 1 pulsations also suggest a ponderomotive redistribution of the background plasma.

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.

Solar cycle change of Pc1 waves observed by an equatorial satellite and on the ground

Abstract Ion cyclotron wave (Pc1 pulsation) activity on the ground has been found to depend strongly on solar activity: Pc1 pulsations occur considerably more often during solar minimum than solar maximum conditions. We have now studied Pc1 wave activity observed by the AMPTE/CCE satellite close to the equatorial source region of these waves, and simultaneously on the ground at Sodankyla, Finland, during two periods of contrasting solar activity levels: August 1985 – May 1986 (with average daily sunspot number of 13.9) and May 1988 – January 1989 (118.2). The total occurrence rate of waves in space decreased by a factor 1.6 from low to high sunspot times, in good agreement with a simultaneous decrease observed for high-latitude unstructured Pc1s on ground. The structured Pc1s (classical pearls) suffer a greater depletion in high sunspot times. The data suggests that the long-term variation of high-latitude Pc1s may entirely be due to a change in wave generation. Another, holistic view on wave appearance in a given flux tube with several factors affecting the observed changes is also possible. Ionospheric ducting does not seem to be a significant factor in regulating long-term activity of high-latitude Pc1s. The data also suggests a nonlinear dependence between Pc1 activity and solar activity as measured by sunspot mumbers.

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.

Riometer absorption events in the evening-to-afternoon sector of the auroral and sub-auroral zone and movements of the IPDP source

Abstract We analyse data from the Finnish north-south chain of riometers and pulsation magnetometers to show that there is a type of electron precipitation event in the aftemoon-to-evening sector of the auroral and sub-auroral zone which occurs simultaneously with IPDP pulsation events. These events show concurrent motions of their sources. We conclude that in these cases riometer absorption events can be used to trace the movements of the source field line of pulsations. This information is used in model calculations which simulate the motions of the IPDP source region by taking into account the geometry of the particle injection boundary in the midnight sector and the westward drift of energetic protons in the magnetosphere.

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.

Fluctuations of the repetition period of Pc1 Pearl pulsations

In this paper we investigate fluctuations of the repetition period of geomagnetic Pc1 pearl pulsations. Starting from calculated repetition period we present a general formula for the deviation of the repetition period as a function of wave frequency and stochastic parameters of the medium along the ray trace. We then apply this formula for a dipole magnetic field with a simple plasma distribution, and show that a linear correlation between repetition period and its deviation is predicted. This correlation and the frequency dependence of fluctuations are then compared with experimental values measured from selected Pc1 pearl events observed in Finland.